CMake: Rename and expose 'USE_EXPERIMENTAL_TESTS' option

The `USE_EXPERIMENTAL_TESTS` variable was not exposed as an option, so
the two tests that use the option to check whether tests should be run
had to be manually enabled by changing `tests/python/CMakeLists.txt`.

This commit renames the variable to `WITH_TESTS_EXPERIMENTAL`, defaults
the option to `OFF`, and marks it as an advanced option.

Pull Request: https://projects.blender.org/blender/blender/pulls/133831

CMakeLists.txt

@@ -864,6 +864,14 @@ else()
   set(WITH_LINUX_OFFICIAL_RELEASE_TESTS OFF)
 endif()
 
+option(WITH_TESTS_EXPERIMENTAL "\
+Run tests marked as experimental. \
+These tests are labeled as such due to long runtime, flakey results, or other issues that make them unsuitable \
+for blocking a build on."
+  OFF
+)
+mark_as_advanced(WITH_TESTS_EXPERIMENTAL)
+
 # Enabled by default for typical use cases to speed up development cycles. However, when looking
 # into threading or memory related issues (in dependency graph, out-of-bounds, etc) forcing single
 # test per Blender instance could give much better clues about the root of the problem.

tests/python/CMakeLists.txt

@@ -4,10 +4,6 @@
 
 # Use '--write-blend=/tmp/test.blend' to view output
 
-# Some tests are interesting but take too long to run
-# and don't give deterministic results
-set(USE_EXPERIMENTAL_TESTS FALSE)
-
 set(TEST_SRC_DIR ${CMAKE_SOURCE_DIR}/tests/data)
 set(TEST_PYTHON_DIR ${CMAKE_SOURCE_DIR}/tests/python)
 set(TEST_OUT_DIR ${CMAKE_BINARY_DIR}/tests)
@@ -228,7 +224,7 @@ add_blender_test(
 )
 
 # test running operators doesn't segfault under various conditions
-if(USE_EXPERIMENTAL_TESTS)
+if(WITH_TESTS_EXPERIMENTAL)
   add_blender_test(
     script_run_operators
     --python ${CMAKE_CURRENT_LIST_DIR}/bl_run_operators.py
@@ -1081,7 +1077,7 @@ if(WITH_GPU_RENDER_TESTS AND TEST_SRC_DIR_EXISTS)
   set(render_tests
   )
 
-  if(USE_EXPERIMENTAL_TESTS)
+  if(WITH_TESTS_EXPERIMENTAL)
     # The viewport tests are flakey and difficult to keep up to date for use
     # in a CI environment due to constantly changing UI, but may still be helpful
     # for local development.