Try to improve incremental build times for tests

[arxanas]

The goal is to improve incremental test times.

Before:

$ hyperfine --warmup 3 --prepare 'echo "// @nocommit test" >>git-branchless/tests/command/test_amend.rs' 'cargo test --test mod --no-run'   
Benchmark 1: cargo test --test mod --no-run
  Time (mean ± σ):      6.927 s ±  0.123 s    [User: 7.652 s, System: 1.738 s]
  Range (min … max):    6.754 s …  7.161 s    10 runs

After:

$ hyperfine --warmup 3 --prepare 'echo "// @nocommit test" >>git-branchless/tests/test_amend.rs' 'cargo test --test test_amend --no-run'   
Benchmark 1: cargo test --test test_amend --no-run
  Time (mean ± σ):      1.771 s ±  0.012 s    [User: 1.471 s, System: 0.330 s]
  Range (min … max):    1.750 s …  1.793 s    10 runs

---

The cargo test no-op time is pretty high (~500ms), which set an unfortunate lower bound on incremental test time:

$ hyperfine --warmup 3 'cargo test --test test_amend --no-run'
Benchmark 1: cargo test --test test_amend --no-run
  Time (mean ± σ):     466.1 ms ±   5.9 ms    [User: 321.7 ms, System: 109.5 ms]
  Range (min … max):   455.9 ms … 475.6 ms    10 runs