Unittests: 'versioning' py test: generate multiple instances to parallelize it.

The `io_blendfile_versioning` test is currently one of the slowest
(excluding Cycles ones) in debug builds, it can easily take several
minutes to complete.

This commit split it into several instances, each processing a subset of
all the blendfiles.

This gives a strong speed-up when only running that specific test.
As expected, speedup is neglectable when running the whole test suite
though.

| instances | debug  | release | debug all* | release all |
| --------- | ------ | ------- | ---------- | ----------- |
|         1 | 190.95 |   19.39 |     439.54 |       63.51 |
|         4 |  61.80 |    6.81 |        N/A |         N/A |
|         8 |  38.33 |    5.14 |     435.00 |       58.93 |
|        16 |  33.97 |    4.16 |        N/A |         N/A |
|        32 |  46.54 |    5.14 |        N/A |         N/A |

Times are in seconds.
`instances` are the number of tests generated (1 is same as before this
commit).
The first two columns are timings for running the versioning test only,
the last two are timings for the full test suite (excluding Cycles tests
in the debug build case).

tests/python/CMakeLists.txt

@@ -211,14 +211,23 @@ add_blender_test(
   --output-dir ${TEST_OUT_DIR}/blendfile_io/
 )
 
-add_blender_test(
-  blendfile_versioning
-  --log "*blendfile*"
-  --debug-memory
-  --debug
-  --python ${CMAKE_CURRENT_LIST_DIR}/bl_blendfile_versioning.py --
-  --src-test-dir ${TEST_SRC_DIR}/
-)
+# This test can be extremely long, especially in debug builds.
+# Generate BLENDFILE_VERSIONING_SPLIT_RANGE instances of the test,
+# each processing their own subset of the whole set of blendfiles.
+set(BLENDFILE_VERSIONING_SPLIT_RANGE 8)
+math(EXPR BLENDFILE_VERSIONING_SPLIT_RANGE_CMAKE "${BLENDFILE_VERSIONING_SPLIT_RANGE} - 1")
+foreach(idx RANGE ${BLENDFILE_VERSIONING_SPLIT_RANGE_CMAKE})
+  add_blender_test(
+    "blendfile_versioning_${idx}_over_${BLENDFILE_VERSIONING_SPLIT_RANGE}"
+    --log "*blendfile*"
+    --debug-memory
+    --debug
+    --python ${CMAKE_CURRENT_LIST_DIR}/bl_blendfile_versioning.py --
+    --src-test-dir ${TEST_SRC_DIR}/
+    --slice-range ${BLENDFILE_VERSIONING_SPLIT_RANGE}
+    --slice-index ${idx}
+  )
+endforeach()
 
 add_blender_test(
   blendfile_liblink

tests/python/bl_blendfile_versioning.py

@@ -43,6 +43,14 @@ class TestBlendFileOpenAllTestFiles(TestHelper):
             "ram_glsl.blend",
         }
 
+        # Generate the slice of blendfile paths that this instance of the test should process.
+        blendfile_paths = [p for p in self.iter_blendfiles_from_directory(self.args.src_test_dir)]
+        # `os.scandir()` used by `iter_blendfiles_from_directory` does not
+        # guarantee any form of order.
+        blendfile_paths.sort()
+        slice_indices = self.generate_slice_indices(len(blendfile_paths), self.args.slice_range, self.args.slice_index)
+        self.blendfile_paths = blendfile_paths[slice_indices[0]:slice_indices[1]]
+
     @classmethod
     def iter_blendfiles_from_directory(cls, root_path):
         for dir_entry in os.scandir(root_path):
@@ -52,12 +60,20 @@ class TestBlendFileOpenAllTestFiles(TestHelper):
                 if os.path.splitext(dir_entry.path)[1] == ".blend":
                     yield dir_entry.path
 
+    @staticmethod
+    def generate_slice_indices(total_len, slice_range, slice_index):
+        slice_stride_base = total_len // slice_range
+        slice_stride_remain = total_len % slice_range
+        gen_indices = lambda i: (
+            (i * (slice_stride_base + 1))
+            if i < slice_stride_remain else
+            (slice_stride_remain * (slice_stride_base + 1)) + ((i - slice_stride_remain) * slice_stride_base)
+        )
+        slice_indices = [(gen_indices(i), gen_indices(i + 1)) for i in range(slice_range)]
+        return slice_indices[slice_index]
+
     def test_open(self):
-        blendfile_paths = [p for p in self.iter_blendfiles_from_directory(self.args.src_test_dir)]
-        # `os.scandir()` used by `iter_blendfiles_from_directory` does not
-        # guarantee any form of order.
-        blendfile_paths.sort()
-        for bfp in blendfile_paths:
+        for bfp in self.blendfile_paths:
             if os.path.basename(bfp) in self.excluded_paths:
                 continue
             bpy.ops.wm.read_homefile(use_empty=True, use_factory_startup=True)
@@ -84,12 +100,34 @@ def argparse_create():
         required=False,
     )
 
+    parser.add_argument(
+        "--slice-range",
+        dest="slice_range",
+        type=int,
+        default=1,
+        help="How many instances of this test are launched in parallel, the list of available blendfiles is then sliced "
+             "and each instance only processes the part matching its given `--slice-index`.",
+        required=False,
+    )
+    parser.add_argument(
+        "--slice-index",
+        dest="slice_index",
+        type=int,
+        default=0,
+        help="The index of the slice in blendfiles that this instance should process."
+             "Should always be specified when `--slice-range` > 1",
+        required=False,
+    )
+
     return parser
 
 
 def main():
     args = argparse_create().parse_args()
 
+    assert(args.slice_range > 0)
+    assert(0 <= args.slice_index < args.slice_range)
+
     for Test in TESTS:
         Test(args).run_all_tests()