Tests should be spread out from fortitude/tests/check.rs

[LiamPattinson]

This file is getting quite unwieldy and hard to navigate, and it would be better to break it up.

For me personally, the combination of a large amount of Rust, Fortran, and inline test snapshots within a single file is also confusing my text editor and breaking syntax highlighting (although I'm finally migrating over to VSCode, which seems to handle it just fine).