Live/incremental testing

Further to discussion on Zulip a while back, this issue is to track what's required to achieve "Live/incremental testing" for Rust (à la Wallaby.js and Visual Studio); the goal is for RA to run only those tests that have been affected by edits and display the results inline.

The broad design in mind for this is as follows:

• tests are compiled with instrumentation that can determine the set of code regions^* that each exercises
• initial run of all tests constructs two maps:
  • one from each code region to the set of tests that cover it; and
  • one from each test to both its result (including point of failure) and the set of code regions that it covers
• each code region displays some visual indication of the aggregate results of its covering tests, with points of failure clearly indicated
• when a code region is modified^**, all its covering tests are queued for execution
• when a test is executed, the maps and visual indications are updated

^* The ultimate solution would probably instrument at the level of basic blocks, eg via -Zinstrument-coverage—but this (presently) requires LLVM, whereas cg-clif is probably a better fit for this feature overall (e.g. because hot-swapped JIT can help minimise compilation times); instrumenting at the function level may be more easily achievable at the outset.

^** The ultimate solution would probably enqueue affected tests as code is being edited—but this requires virtual files to be shared from RA to rustc for compilation, and currently rustc is not called in-process; queuing affected tests on save may be more achievable at the outset.

On the UI front, it's probably worth noting that VSCode may get some sort of API for precisely this over the next couple of months.

Start with the linked Zulip discussion and the referenced rustc_codegen_cranelift issues on coverage instrumentation and hot-swapped JIT. Define the scope around tracking code-region/test coverage, rerunning affected tests, and displaying inline results; the issue does not name implementation files or tests.