Move AbstractKeywordValidatorTest

It allows not to use @VisibleForTesting (and public, at that) in
KeywordDescriptor.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Modify KeywordDescriptor a bit more, introduce AbstractKeywordEquivalenceTest

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>