Update Ahem capitalization in LayoutTests

Update shorthand font declarations to use Ahem instead of ahem in tests.
This is a follow up to r174325 where font-face declarations where
changed in the same way. This change is due to the Windows DW port
being case sensetive for side loaded fonts used by content_shell.

BUG=333029
TBR=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/291823004

git-svn-id: svn://svn.chromium.org/blink/trunk@174440 bbb929c8-8fbe-4397-9dbb-9b2b20218538