Undo all-parents memoization in test

yas-reload-all does not undo the memoization done in yas--all-parents, which
causes the issue-492-and-494 test to fail when run after any tests that
expands yas-with-some-interesting-snippet-dirs.

With the memoization enabled, c++-mode is considered a parent of c-mode,
causing the issue-492-and-494 test to fail.  With the memoization reset,
this does not occur.

yasnippet-tests.el

@@ -1438,6 +1438,7 @@ hello ${1:$(when (stringp yas-text) (funcall func yas-text))} foo${1:$$(concat \
   (yas-with-snippet-dirs '((".emacs.d/snippets"
                             ("yas--test-mode")))
     (yas-reload-all)
+    (put 'c-mode 'yas--all-parents nil)
     (with-temp-buffer
       (let* ((major-mode 'yas--test-mode)
              (expected `(fundamental-mode