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).

test/lisp/filenotify-tests.el

@@ -266,9 +266,8 @@ This returns only for the local case and gfilenotify; otherwise it is nil.
   (declare (indent 1))
   `(ert-deftest ,(intern (concat (symbol-name test) "-remote")) ()
      ,docstring
-     :tags '(:expensive-test)
      :expected-result (or ,expected :passed)
-     (skip-unless (not ,skip))
+     :tags ,(if skip '(:expensive-test :unstable) '(:expensive-test))
      (let* ((temporary-file-directory
 	     file-notify-test-remote-temporary-file-directory)
 	    (ert-test (ert-get-test ',test))

test/lisp/net/tramp-tests.el

@@ -4579,7 +4579,7 @@ This tests also `make-symbolic-link', `file-truename' and `add-name-to-file'."
 	      ;; skip the test then.
 	      (condition-case nil
 		  (vc-create-repo (car vc-handled-backends))
-		(error (skip-unless nil)))
+		(error (ert-skip "`vc-create-repo' not supported")))
 	      ;; The structure of VC-FILESET is not documented.  Let's
 	      ;; hope it won't change.
 	      (condition-case nil

test/lisp/progmodes/python-tests.el

@@ -5350,7 +5350,7 @@ buffer with overlapping strings."
   ;; The description of the problem it's trying to catch is not clear enough
   ;; to be able to see if the underlying problem is really fixed, sadly.
   ;; E.g. I don't know what is meant by "overlap", really.
-  (skip-unless nil)
+  :tags '(:unstable)
   (python-tests-with-temp-buffer "''' '\n''' ' '\n"
     (syntax-propertize (point-max))
     ;; Create a situation where strings nominally overlap.  This