`stubtest` does not validate incomplete features

[sobolevn]

Similar to #17628 stubtest allows a configuration file with enable_incomplete_feature = setting.

But, it does not validate it, while main.py does that:

mypy/mypy/main.py

Lines 1340 to 1345 in fe4df12

	# Validate incomplete features.
	for feature in options.enable_incomplete_feature:
	if feature not in INCOMPLETE_FEATURES | COMPLETE_FEATURES:
	parser.error(f"Unknown incomplete feature: {feature}")
	if feature in COMPLETE_FEATURES:
	print(f"Warning: {feature} is already enabled by default")

So, right now you can specify wrong incomplete features or complete ones with no errors / warnings.