Add test framework for OptionsBase classes and their Converters.

Because OptionsBase implements equals() as a final method, subclasses can
only add fields in certain ways for OptionsBase to properly obey equals()
semantics. Specifically, all fields must be public and @Option annotated.
The OptionsTester checks for these two things.

Additionally, Converters must make sure to always return equals() values
on equals() (or equivalent) input. The OptionsTester includes a check that
all Converters named by the OptionsBase subclass being tested have matching
ConverterTesters, and if valid default values are specified (i.e., on
Options which are not multi-valued or default null), that these defaults
are among the values tested by the ConverterTesters.

The ConverterTesters themselves are wrapped EqualsTesters, testing that
the output of a Converter obeys equals() as expected for the same input
(or equivalent ones), and is consistent across calls to the same Converter
instance or different Converter instances.

Between these two, OptionsBase subclasses can have reasonable certainty
that two instances of themselves which were parsed equally - or underwent
equivalent transformations - will be equal.

This does not actually test any OptionsBase subclasses or Converter
implementations; it merely adds a framework. Future changes will cover
automatically testing all of the OptionsBase subclasses in a
RuleClassProvider, but naturally, this requires writing test data for
each Converter in the Bazel codebase first.

RELNOTES: None.
PiperOrigin-RevId: 162522445

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