test: cover each comparison branch of each CE comparator

The tests are required to perform safe refactoring of comparators.

Covered comparison functions are:
- `tuple_compare_sequential`: all valid `is_nullable` and
  `has_optional_parts` combinations.
- `tuple_compare_with_key_sequential`: all valid `is_nullable` and
  `has_optional_parts` combinations.
- `tuple_compare_slowpath`: all valid `is_nullable` and
  `has_optional_parts` combinations. `has_json_paths` and
  `is_multikey` options are not covered since they don't affect
  the comparison logic.
- `tuple_compare_with_key_slowpath`: all valid `is_nullable` and
  `has_optional_parts` combinations. See the point above about
  other options.
- `key_compare`: both `is_nullable` option variants.

NO_DOC=new tests
NO_CHANGELOG=new tests