this greatly speeds up the time to run all tests, at the cost of slightly larger recompile times for individual tests.
this unfortunately adds the requirement that all tests are listed in `runner.rs` for the crate.
to avoid forgetting, i've added a new test that ensures the directory is in sync with the file.
## benchmarks
before this change, recompiling all tests took 32-50 seconds and running a single test took 3.5 seconds:
```
; hyperfine 'touch lib/src/lib.rs && cargo t --test test_working_copy'
Time (mean ± σ): 3.543 s ± 0.168 s [User: 2.597 s, System: 1.262 s]
Range (min … max): 3.400 s … 3.847 s 10 runs
```
after this change, recompiling all tests take 4 seconds:
```
; hyperfine 'touch lib/src/lib.rs ; cargo t --test runner --no-run'
Time (mean ± σ): 4.055 s ± 0.123 s [User: 3.591 s, System: 1.593 s]
Range (min … max): 3.804 s … 4.159 s 10 runs
```
and running a single test takes about the same:
```
; hyperfine 'touch lib/src/lib.rs && cargo t --test runner -- test_working_copy'
Time (mean ± σ): 4.129 s ± 0.120 s [User: 3.636 s, System: 1.593 s]
Range (min … max): 3.933 s … 4.346 s 10 runs
```
about 1.4 seconds of that is the time for the runner, of which .4 is the time for the linker. so
there may be room for further improving the times.
When the user doesn't have a configured default command, we show a
hint saying to set `ui.default-command`. I think the user is very
likely to want to set that in the user-wide config, so let's include
the command in the hint.
It should be better to handle invalid -R path globally. The error message is
a bit worse, but I think it's still okay.
This helps to load temporary config from the cwd-relative path. If the command
processing continued with an invalid -R path, the temporary config would have
to be explicitly discarded.
