Port test-suite.md to rustbuild

[rust-lang]

Port test-suite.md to rustbuild

[jacwah]

How to run the test suite is mostly covered on the x.py page. I think a lot of the recipes can replaced by a link there. Maybe test-suite.md can focus more on creating and editing tests instead.

[est31]

Creating and editing tests is described inside the rust tree itself: https://github.com/rust-lang/rust/blob/master/src/test/COMPILER_TESTS.md

So dunno... maybe replace the page with links to those two places?

[pietroalbini]

There isn't any content on that page anymore.