This simplifies some [Fact] tests that iterate over tablet configurations into [Theory] tests, properly making individual tests per tested object. This should also speed up the time it takes to run the full test suite, as configurations should only be evaluated a single time per test start, instead of on every individual [Fact] test start.

Rewritten the following tests to use [Theory] with [MemberData]:

• Configurations_Have_ExistentParsers
• Configurations_Verify_Configs_With_Schema
• Configurations_Are_Linted

It also flips the diff side of Configurations_Are_Linted as this previously indicated the more useless side of the diff.

It also changes the object types to be of TheoryData<T> instead of IEnumerable<object[]?>, as suggested in more recent xunit documentation.