`core_ext/string/access.rb` test what we are documenting.

I also extracted the tests from a single bulk method into a separate test-case.
The new tests cover the API described in the docs.

There are two skipped tests, which are broken as of 2ef1fb2c455ca53a0c1e1768f50824926ce28bd3
* #to with negative Fixnum, position is counted from the end
* #from and #to can be combined

This was brought to my attention by #13627.

Closes #13627.