Improve handling of locales in the test suite.

Improve handling of locales in the test suite.

* test-suite/guile-test (run-tests): Load each test file within
  (with-locale "C" ...).

* test-suite/tests/encoding-iso88591.test:
* test-suite/tests/encoding-iso88597.test:
* test-suite/tests/encoding-utf8.test:
* test-suite/tests/srfi-14.test: Remove broken code to save and restore
  the previous locale.

* test-suite/tests/bytevectors.test:
* test-suite/tests/format.test:
* test-suite/tests/regexp.test:
* test-suite/tests/srfi-19.test:
* test-suite/tests/tree-il.test: Make sure 'setlocale' is defined before
  calling it.