Tests intermittently fail with timeouts on CI

[sourcefrog]

Sometimes tests fail in GitHub actions due to timeouts, e.g. https://github.com/sourcefrog/cargo-mutants/runs/7932363158?check_suite_focus=true.

It's not always the same test. It might be more common on Windows but it does happen elsewhere.

I think this is because the CI machines are over-committed and sometimes slow down.

Options in order of preference:

• Add more debug logging about timeouts that's visible in CI, to help understand the cause. (Maybe send trace logs to stdout?)
• Take the minimum timeout from an environment variable that can be overridden in CI.
• Set a longer timeout specifically during cargo-mutants' own test suite?
• Increase the default minimum timeout from 20s (although that already seems like a very long time for these simple tests.)
• Just ignore the flakes and live with it (not great).

It's possible that this is not just slow but actually somehow getting deadlocked?

failures:

---- cargo_mutants_in_replace_dependency_tree_passes stdout ----
thread 'cargo_mutants_in_replace_dependency_tree_passes' panicked at 'Unexpected failure.
code-3
stderr=```""```
command=`"D:\\a\\cargo-mutants\\cargo-mutants\\target\\debug\\cargo-mutants.exe" "mutants" "--no-times" "--no-copy-target" "--no-shuffle" "-d" "testdata/tree/replace_dependency"`
code=3
stdout="Copy source to scratch directory ... done\nUnmutated baseline ... ok\nFound 2 mutants to test\nsrc/lib.rs:6: replace is_even -> bool with false ... TIMEOUT\n2 mutants tested: 1 caught, 1 timeouts\n"
stderr=""
', /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f\library\core\src\ops\function.rs:248:5
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library\std\src\panicking.rs:584
   1: core::panicking::panic_fmt
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library\core\src\panicking.rs:142
   2: core::panicking::panic_display<assert_cmd::assert::AssertError>
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f\library\core\src\panicking.rs:72
   3: assert_cmd::assert::AssertError::panic<assert_cmd::assert::Assert>
             at C:\Users\runneradmin\.cargo\registry\src\github.com-1ecc6299db9ec823\assert_cmd-2.0.4\src\assert.rs:1036
   4: core::ops::function::FnOnce::call_once<assert_cmd::assert::Assert (*)(assert_cmd::assert::AssertError),tuple$<assert_cmd::assert::AssertError> >
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f\library\core\src\ops\function.rs:248
   5: enum$<core::result::Result<assert_cmd::assert::Assert,assert_cmd::assert::AssertError> >::unwrap_or_else<assert_cmd::assert::Assert,assert_cmd::assert::AssertError,assert_cmd::assert::Assert (*)(assert_cmd::assert::AssertError)>
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f\library\core\src\result.rs:1484
   6: assert_cmd::assert::Assert::success
             at C:\Users\runneradmin\.cargo\registry\src\github.com-1ecc6299db9ec823\assert_cmd-2.0.4\src\assert.rs:156
   7: cli::cargo_mutants_in_replace_dependency_tree_passes
             at .\tests\cli.rs:906
   8: cli::cargo_mutants_in_replace_dependency_tree_passes::closure$0
             at .\tests\cli.rs:901
   9: core::ops::function::FnOnce::call_once<cli::cargo_mutants_in_replace_dependency_tree_passes::closure_env$0,tuple$<> >
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f\library\core\src\ops\function.rs:248
  10: core::ops::function::FnOnce::call_once
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library\core\src\ops\function.rs:248
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.


failures:
    cargo_mutants_in_replace_dependency_tree_passes

test result: FAILED. 54 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in [143](https://github.com/sourcefrog/cargo-mutants/runs/7932363158?check_suite_focus=true#step:6:144).12s

[sourcefrog]

This might be now fixed with an increased time limit and also moving them to separate workspaces, which might allow more parallelism.