Prefer skip-unless in more tests

* test/lisp/emacs-lisp/timer-tests.el (timer-tests-debug-timer-check):
* test/src/decompress-tests.el (zlib--decompress):
* test/src/xml-tests.el (libxml-tests): Prefer skip-unless.
1 job for master in 55 minutes and 21 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #10066
test-all

00:55:21