[TKW] Bug: cache tests fail if cache already exists

[GMNGeoffrey]

The cache tests are supposed to reset the cache before they run by calling reset_cache_manager, but that method just returns early if the cache hasn't been initialized without deleting the cache directory:

iree-turbine/iree/turbine/kernel/wave/cache.py

Lines 269 to 275 in bca982a

	def reset_cache_manager() -> WaveCacheManager:
	if not "_global_cache_manager" in globals():
	return
	if os.path.exists(CACHE_BASE_DIR):
	shutil.rmtree(CACHE_BASE_DIR)
	global _global_cache_manager
	del _global_cache_manager

so if you run them once and create a cache entry and then run them again, the check that the first call wasn't cached fails:

iree-turbine/tests/kernel/wave/runtime/cache_test.py

Lines 207 to 209 in bca982a

	assert isinstance(
	mb, tk.compiler.builder.ModuleBuilder
	), "Expected first call to not be cached."

[GMNGeoffrey]

Either reset_cache_manager should delete the cache directory regardless, the tests should first initialize the cache manager before resetting it (seems a bit weird), or there should be a separate method for clearing the cache independent of resetting the cache manager. Given that the cache tests are the only users of reset_cache_manager AFAICT, I think option 1 makes the most sense.

[GMNGeoffrey]

It's also maybe a bit aggressive to have the cache tests reset someone's entire cache. Maybe they should use a private cache in a temporary directory instead? That solves both problems.