Fix out-of-bound array accesses in libstdc++ testsuite

Fix out-of-bound array accesses in libstdc++ testsuite

	* testsuite/21_strings/basic_string/modifiers/append/char/1.cc: Fix
	reads past the end of strings.
	* testsuite/21_strings/basic_string/operations/compare/char/1.cc:
	Likewise.
	* testsuite/21_strings/char_traits/requirements/short/1.cc: Fix
	invalid array accesses.
	* testsuite/experimental/string_view/operations/compare/char/1.cc:
	Fix read past the end of string.
	* testsuite/util/testsuite_character.cc: Fix out-of-bounds write.

From-SVN: r238609