Use `with-latin1-locale' in `regexp.test'.

Use `with-latin1-locale' in `regexp.test'.

As a side effect, it fixes tests on platforms with no 8-bit locale and
where executing regexps on characters >= 128 can lead to errors such as
`cannot convert to output locale "US-ASCII": ""\x80""'.

This commit partially reverts 7583976b ("More setlocale robustness in
regexp tests").

* test-suite/tests/regexp.test (mysetlocale, set-latin-1): Remove.
  ("regexp-quote"): Use `with-latin1-locale' instead of the above
  procedures.