[SPARK-49266][SQL][TESTS] Add collation support unit tests for FindInSet

### What changes were proposed in this pull request?
Add collation support unit tests for:

- FindInSet

This PR contains test-only changes, providing additional test coverage for cases such as:
- case and accent variation
- one-to-many case mapping
- conditional case mapping
- surrogate pairs
- etc.

### Why are the changes needed?
Improve collation support testing.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
New unit tests in `CollationSupportSuite`.

### Was this patch authored or co-authored using generative AI tooling?
Yes.

Closes apache#47728 from uros-db/unit-tests-4.

Authored-by: Uros Bojanic <[email protected]>
Signed-off-by: Max Gekk <[email protected]>