Don't presume existence or success of setlocale in test-suite

Don't presume existence or success of setlocale in test-suite

* test-suite/lib.scm (with-locale, with-locale*): new test functions

* test-suite/tests/encoding-escapes: don't fail if en_US.utf8 doesn't exist

* test-suite/tests/encoding-iso88591.test: set and restore locale, if
  possible

* test-suite/tests/encoding-iso88597.test: set and restore locale, if
  possible

* test-suite/tests/encoding-utf8.test: set and restore locale, if possible

* test-suite/tests/srfi-14.test: don't need to setlocale to Latin-1 to
  test Latin-1 since string conversion is handled at read/compile time.
  Set and restore locale, if possible.