When this option is specified (which it is when running in CI), stubtest won't run when a platform is not specified in METADATA.toml. The test is marked as "skipped". When it's not specified (the default), stubtest will run and will most likely case an error.

I noticed this when running stubtest_third_party.py locally on my Linux system and it errored on pywin32.

I suggest we either:

1. Remove that option altogether and always skip stubtest for stubs not specified in METADATA.toml; or
2. reverse the logic so that those tests are skipped by default and only generate errors when an explicit option is given.