Add tests for font-variant-ligatures: discretionary-ligatures and historical-ligatures

This is now possible to test with recent fonts added to the layout tests.

BUG=

Review URL: https://codereview.chromium.org/186273003

git-svn-id: svn://svn.chromium.org/blink/trunk@168681 bbb929c8-8fbe-4397-9dbb-9b2b20218538