Fix test that breaks on CI

[neocotic]

While running builds for 0.5.0 I noticed that one of the tests for the media-queries.svg fixture was failing, but only for PNG output and only on CI. Annoyingly, it was working fine locally for Node.js v8, v10, and v11, however, the CI job for Node.js v8 was the only one passing.

Tests for this fixture have been skipped for now as a temporary measure, however, I'd like to avoid doing this for obvious reasons and investigate the root cause.

I might see if I can spin up a Ubuntu Trusty (the version used by Travis) VM locally and try to replicate there as I'm on Ubuntu Cosmic and cannot replicate. I haven't tried in any other OS' though.

[neocotic]

I just noticed that Ubuntu Xenial is now available on Travis, so I might try using that instead of Trusty to see if that has any affect.

[neocotic]

Moved away from Travis to GitHub workflows after they started demanding money. However, there are some tests still marked as skipped due to inconsistent results that should be investigated.