internal: Add run-tests command
This command is similar to `cargo test` except that it uses r-a to run tests instead of compiling and running them with rustc. This is slower than `cargo test` and it is only useful for me to see a bird view of what needs to be fixed. The current output is:
```
48 passed, 5028 failed, 2 ignored
All tests 174.74s, 648ginstr
```
48 is very low, but higher than what I originally thought.
Now that there is some passing tests, I can show the plan:
https://github.com/rust-lang/rust-analyzer/assets/45197576/76d7d777-1843-4ca4-b7fe-e463bdade6cb
That is, at the end, I want to be able to immediately re run every test after every change. (0.5s is not really immediate, but it's not finished yet, and it is way better than 8s that running a typical test in r-a will take on my system)
3a4f9a1416