Addendum to bug #37339 : make the test case portable to windows

by using and taking out a full path.

mysql-test/r/ctype_filesystem.result

 SET CHARACTER SET utf8;
 SHOW VARIABLES like 'character_sets_dir';
 Variable_name	Value
-character_sets_dir	/ß/
+character_sets_dir	MYSQL_TEST_DIR/ß/
 SHOW VARIABLES like 'character_set_filesystem';
 Variable_name	Value
 character_set_filesystem	latin1

mysql-test/t/ctype_filesystem-master.opt

---character-sets-dir=/ß
+--character-sets-dir=$MYSQL_TEST_DIR/
 --character-set-filesystem=latin1

mysql-test/t/ctype_filesystem.test

 SET CHARACTER SET utf8;
+--replace_result $MYSQL_TEST_DIR MYSQL_TEST_DIR
 SHOW VARIABLES like 'character_sets_dir';
 SHOW VARIABLES like 'character_set_filesystem';
 SHOW VARIABLES like 'character_set_client';