Add full support for the `litmus7` tool

[MattWindsor91]

Like herd7, litmus7 checks memory-model behaviour of assembly programs; unlike herd7, it does so by running the program a very large number of times on an actual machine. This means that its results are not guaranteed to be complete.

Supporting it is useful for three use cases:

• checking whether the actual memory order behaviour of a machine matches the modelled behaviour according to herd7;
• investigating assembly that Herd doesn't yet support;
• investigating assembly with fewer sanitiser passes than we need for Herd, which might be a bit more faithful to the original compilation (and helps us debug issues in the passes themselves).

It is not useful for getting a C11-compliant execution set from C, since it'll just compile the C down to assembly!

At time of writing, act has very rudimentary support for litmus in its litmusify mode. This issue tracks completing that support.

• [x] Add (and test) remote invocation of litmus on other machines
• [ ] Add results scraping
• [ ] Connect litmus to test mode as an optional backend

[MattWindsor91]

See #81 for a possible bug related to litmus7.

[MattWindsor91]

I think this is, more or less, done now.