Enable declarations to be marked as test support only

[Chuck]

Then when -include-tests:no, these declarations would be excluded from code generation. Of course, if non-excluded code references them, that should be an error.

class TestSupportAttribute
	has AttributeUsage(AttributeTargets.All, allowMultiple=false)
	inherits Attribute
	"""
	Declarations marked as `TestSupport` will be excluded when the compiler is passed -include-tests:no.
	"""
	pass

Cobra itself would benefit from this feature.