CLN: tests for str.cat
Cleaning up some left-overs from #20347 and preparing the tests for changing the default of `join` to `'left'` in 1.0 (which would break some tests that assume no alignment currently). As a side benefit, this has:
* nicer parametrization
* nicer switches (and reduced usage of them within the tests)
* now follows `result / expected` pattern instead of calculating result within `assert_..._equal`