Skip custom-elements/registering-custom-elements/unresolved-element-pseudoclass

When we include tests from the  above directory in the w3c web-platform-tests
repo in our checkout, we can end up exceeding the 260 character path
limitation on windows and much sadness ensues.

TBR=tonyg@chromium.org
BUG=405274

Review URL: https://codereview.chromium.org/490673005

git-svn-id: svn://svn.chromium.org/blink/trunk@180609 bbb929c8-8fbe-4397-9dbb-9b2b20218538