Parametrize PatternConstraint subclass tests

Replace 16 individual valid/invalid test methods with two parametrized
tests driven by PATTERN_CONSTRAINT_CASES. Covers all 8 PatternConstraint
subclasses: LanguageTag, CountryCodeAlpha2, RegionCode, WikidataId,
PhoneNumber, HexColor, NoWhitespace, SnakeCase.

Moved SnakeCaseConstraint tests from TestErrorHandling (where they were
misplaced) into the parametrized data.

Non-PatternConstraint tests remain as standalone methods: base
PatternConstraint (custom pattern), StrippedConstraint, and
JsonPointerConstraint (empty-string special case).

Author: sethfitz