Add more tests

This MR collects a number of testsuite fixes.

The more noteworthy ones are:

• Split tests between various features (small caps, variations, font options), so it is clearer which parts are failing on which backend
• Make assertions nonfatal. This lets us see more test results, even if one test failed
• Skip tests using diff if diff isn't found. This turns a bunch of failures on msvc into skips

After this, there are a few actual test failures that will need to be addressed eventually:

• The linebreaking tests are broken by glib main's Unicode 16 update, that affects all platforms where we use glib main as a subproject
• The macos backend doesn't support font variations, causing a few tests to fail
• The win32 backend doesn't support font variations, causing a few tests to fail