Use assert for tests instead of visual output compare

[eltos]

To ease maintainability especially for new typst versions, and since the library does not actually produce visual output, the tests should use logical assertions rather than visual output comparison.

Motivation: When updating typst, few pixels will often change, causing the tests to fail, even though the output is still a "true". See #55 for an example.
It will also allow to test against multiple typst versions via CI matrix strategy.

This is just a start, mainly for tracking and as a reminder, the PR is far from complete.

[tingerrr]

I don't quite remember, but I think the ci-semi-stable version of might already support compile-only tests. Otherwise, it may just be appropriate to switch to Tytanic for testing once 0.2.0 is out (should be in tandem with Typst 0.13.0).