Write parse tests for linear gradients

[canova]

We have parse tests for radial gradients' parsing. Linear gradients are simpler than radial ones and probably require lesser tests.
To add parse tests, you can add a new test function to tests/unit/style/parsing/image.rs and with assert_roundtrip_with_context! macro, compare the left gradient with right parsed ones. Implementation is pretty much same with radials except examples.

Linear gradient examples can be found at: https://drafts.csswg.org/css-images/#funcdef-linear-gradient

To run test you can use ./mach test-unit -p style command.

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[AVGP]

I'd like to give this a go :)

[jdm]

@AVGP Please do! Ask questions here if anything is unclear!

[AVGP]

What's up with Travis? Is this DNS-DDoS fallout?

[canova]

Yeah, probably because of DNS attack. Also highfive is not working properly.