Replace (skip-unless nil) by tag :unstable in test packages

* test/lisp/filenotify-tests.el (file-notify--deftest-remote):
Use tag :unstable if SKIP is non-nil.

* test/lisp/net/tramp-tests.el (tramp-test36-vc-registered):
Use ert-skip instead of (skip-unless nil).

* test/lisp/progmodes/python-tests.el
(python-tests--python-nav-end-of-statement--infloop): Use tag
:unstable instead of (skip-unless nil).
2 jobs for master in 60 minutes and 1 second (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #1331
test-all

01:00:01

failed #1332
test-filenotify-gio

00:37:29

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/international/ccl-tests.log
ELC lisp/international/mule-tests.elc
GEN lisp/international/mule-tests.log
ELC lisp/international/mule-util-tests.elc
GEN lisp/international/mule-util-tests.log
ELC lisp/international/ucs-normalize-tests.elc
GEN lisp/international/ucs-normalize-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-f100a208 ...
ERROR: Job failed: execution took longer than 1h0m0s seconds
failed
test-filenotify-gio Test
  GEN      lisp/filenotify-tests.log
Symbol's function definition is void: :expensive-test
Makefile:182: recipe for target 'lisp/filenotify-tests.log' failed
make[1]: *** [lisp/filenotify-tests.log] Error 255
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:247: recipe for target 'lisp/filenotify-tests' failed
make: *** [lisp/filenotify-tests] Error 2
make: Leaving directory '/builds/emacs/emacs/test'
ERROR: Job failed: exit code 1