Consider merging `litmusify` and `test`

[MattWindsor91]

It's somewhat confusing that we now have two different workflows for compiling a C litmus test and checking the results: act test, which is multi-file multi-compiler, and act litmusify, which is single-file single-compiler. The two are slowly starting to converge on the same feature set, which makes me wonder if they should be merged at some point down the road.

This'd involve act test gaining the following features, many of which are already planned:

• [ ] Support single-file input
• [ ] Support stopping act test at a stage before comparison (since litmusify supports a whole range of possible outputs and filters)
• [ ] Support litmus7 (see #53)
• [ ] (Optional) Make it easier to use act test with one compiler
• [ ] (Optional) Support sending the intermediate results to temporary files instead of using a pathset

[MattWindsor91]

Dropping this, because act test doesn't exist anymore, and act litmusify no longer compiles anything.