[Gecko Bug 1530048] part 2: Add test to verify that table, td, and th draw borders using 'currentColor'.

[moz-wptsync-bot]

Differential Revision: https://phabricator.services.mozilla.com/D21249

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1530048
gecko-commit: 72127702116cd18973ab501edfcb04bdc4cbe68a
gecko-integration-branch: central
gecko-reviewers: dbaron

[wpt-pr-bot]

Already reviewed downstream.

[dholbert]

Failing check is wpt-firefox-nightly-stability, with some reftest screenshots that are blank, as we've seen elsewhere. Link to test run: https://taskcluster-artifacts.net/cmEA0yo2RZOZldUWpieJWA/0/public/logs/live_backing.log

This is a known issue (seen in #15661 (comment) and #15534 (comment) ) and it has nothing to do with this test, so it shouldn't block this test merging. @jgraham , would you mind force-merging?

[dholbert]

For the record, if it helps for debugging the blank-screenshot issue down the road:
The failing wpt-firefox-nightly-stability testrun had two unexpected failures, both of which happened in the same testrun (one testrun with a failure in each test = 2 unexpected failures). In that testrun:

• The screenshot of the first test (table-border-3q.html) was blank.
• The screenshot of the second test (table-border-3s.html) was correct.
• For both of those, the screenshot of the reference case was a bogus partially-white-partially-transparent image (with a transparent area on the right side) -- here's that screenshot: