Simplify GenericIndexValidationIT a lot

- One use one source of random value generation and isolate logic around this in enum
- Better descriptions of test cases
- Isolate binary search logic in internal class
- Remove shouldEnforceSizeCapSingleValueMixedTypes because it is just a special case for composite with only one slot
- Break out helper methods where possible
- Verify that we have at least one failure and one success on each invokation of `shouldEnforceSizeCapMixedTypes`

NOTE! Biggest change is we no longer test string array with 0 length strings.
Replaced by string array with 1 byte strings. The reason for this is the
probability of generating two equal values go up to significant heights if we
allow string arrays with 0 length strings.