Update regexp slt tests to refactor out string type tests, cleanup

[Omega359]

Is your feature request related to a problem or challenge?

The regexp.slt test file currently contains tests for regexp_count, regexp_like, regexp_match and regexp_replace functions and do simple tests for acceptable of different string types (just utf8 and ut8view currently).

This test file should be refactored in a similar manner to how the string tests were refactored in #12525 to clean up the testing and extract out the test for each function into their own file.

Describe the solution you'd like

A clean and clear set of test files where testing the string types accepted is clear and reuses common queries similar to how the string tests are done.

Describe alternatives you've considered

No response

Additional context

No response

[Omega359]

Follow up to #14449