Fix all of the broken tests

[cyrgani]

This fixes the test failure described in #693 by wrapping the global Context in std::mem::ManuallyDrop to prevent the compiler from dropping it at the beginning of the second test when overwriting it. putting each test into its own file so they each get a fresh Context.

It also adds a missing reference to the doctest of the animation module to fix it and removes the failing doctest of text::Font::set_filter.

Fixes #693.

[cyrgani]

Marking this as a draft as it cannot fix the new error described in #693 (comment).

[cyrgani]

By putting each test function into its own file, they do not reuse any old statics and therefore they succeed.
This should be ready to merge now @not-fl3.

[cyrgani]

Closing in favor of #866.