Auto merge of #508 - Manishearth:stylo-rel, r=larsbergstrom
Auto merge of #508 - Manishearth:stylo-rel, r=larsbergstrom

Add release stylo unit tests

https://github.com/servo/servo/pull/13700 moved some important binding layout tests to test-stylo (they used to be run by the regen script).

Currently test-stylo is only run in debug mode. This runs it in release mode as well.

Since linux-rel-css already contains a `./mach build-geckolib --release`, and `test-stylo` just needs a rebuild of style and geckolib (which takes half a minute), this shouldn't cause much of an issue.

However, linux-rel-css is one of our slowest builders. Perhaps both `build-geckolib --release` and `test-stylo --release` should be put in linux-dev instead?

r? @larsbergstrom

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UK992/saltfs,servo/saltfs,rwaweber/saltfs,ALikhachev/saltfs,UK992/saltfs,ALikhachev/saltfs,rwaweber/saltfs,servo/saltfs,UK992/saltfs,ALikhachev/saltfs,rwaweber/saltfs