Continuation of add StringUtils.substringBeforeLast(String, int)

[Serwios]

Hi, it's continuation of #967 , basically just added tests to @mmariotti PR, please review @arturobernalg @garydgregory

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[Serwios]

Hi, request to review, thanks @garydgregory

[garydgregory]

• I don't think this is needed or that we should cover every combination of before/after/char/int/any/not and so on.
• don't delete existing tests
• whenever I see int for a character, I think about code points, UTF-16, and supplementary characters which the tests here don't cover.

[Serwios]

I didn't delete any tests, just added new one, old test testSubstringBetween_StringString is below my test

If you think it's not needed then I'll close this pr