servo: Merge #17974 - remove testing feature from stylo_tests (from froydnj:remove-testing-feature); r=SimonSapin

`stylo_tests` currently requires a separate version of the `style` crate, compiled with the `testing` feature, so a function testing the size of specified values can be accessed.  With a few tweaks, we can make the information needed for the test available to the `stylo_tests` crate directly, eliminating the need for a separately-compiled `style` crate.

This doesn't matter much for Servo itself (it might make CI times slightly faster?), but Gecko automation/development would like to run `stylo_tests`, and not having to compile two versions of the `style` crate (or have a dead, test-only function hanging around in the `style` crate) would be a win.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] There are tests for these changes

Source-Repo: https://github.com/servo/servo
Source-Revision: a6369149dc5344b2b80a12fca1c43cf99c94fdc9

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : f23cc39c69a996f3d1023f4c00f8d564d4a3e97f