Move more tests into piet-common

[cmyr]

There are a bunch of tests duplicated across various backends, and it makes sense to move these into piet-common.

The only tests that really need to be backend-specific are tests that rely on platform fonts or behaviour related to platform fonts, like things for hit-testing?

There are also a few tests that were removed from piet-common because of cairo toy-text; these include the missing_font_is_missing test.