Silence check-blink-deps errors in web/tests

web/tests can include headers from its own directory.

I'm not sure this is the correct way to fix it.
I tried adding "+." instead, but that didn't seem to work.

Review URL: https://codereview.chromium.org/115373003

git-svn-id: svn://svn.chromium.org/blink/trunk@163899 bbb929c8-8fbe-4397-9dbb-9b2b20218538