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.