Fix css-shapes tests to load ahem as a web font and wait for fonts ready

[LukeZielinski]

The font-related code is in a shared utility file for these tests. Updated it to load Ahem as a web font and wait for fonts.ready before running tests.

Adding these changes to the tests themselves didn't work, presumably because generate_tests is used and it interferes somehow.

[LukeZielinski]

@foolip @gsnedders Could you please take a look?

Scanning over the results, this looks like a general improvement for both Chrome and Firefox. Safari is worse, likely because of the known document.fonts.ready timing issue on WebKit.

One thing that is wrong with this change is that I see some test assertion failures getting returned as harness statuses (example). Any advice on how to address that?

[foolip]

@LukeZielinski a failed assert turning into a harness error means that there's some code running outside of a test "step function". This is usually fixed by adding t.step_func somewhere.

However, here generate_tests is used. To quote the docs:

This is deprecated because it runs all the tests outside of the test functions and as a result any test
throwing an exception will result in no tests being run. In almost all cases, you should simply call test within the loop you would use to generate the parameter list array.

Can you rewrite to avoid generate_tests?

[foolip]

@LukeZielinski also note that Ahem is still installed as a system font in our Safari config, so the document.fonts.ready bug shouldn't affect this, it'll kick in only if we remove the system font.

[foolip]

@LukeZielinski this is conflicting now, can you rebase if this wasn't already fixed in another PR?

[LukeZielinski]

Abandoning, this was fixed properly in #19577. Thanks for the ping.