Clean up expectations for mozilla variables tests

TestExpectations and LeakExpectations contain some expectations for
external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/variables
tests that do not match the current status on the flakiness dashboard.
This patch tightens up expectations to match the current reality.

BUG=582836

Review-Url: https://codereview.chromium.org/2793263002
Cr-Commit-Position: refs/heads/master@{#463181}
2 files changed