Make errors from make_test_list legible

[sourcefrog]

In debugging sourcefrog/cargo-mutants#226 I found that invocations like ~/.rustup/toolchains/beta-x86_64-unknown-linux-gnu/bin/cargo nextest run cause some test failures in the nextest tree, for reasons I'm still working to understand.

In main, those errors are printed like this, with a pile of ASCII byte values:

thread 'basic::test_termination' panicked at nextest-runner/tests/integration/fixtures.rs:383:10:
test list successfully created: CommandFail { binary_id: RustBinaryId("cdylib-example"), command: ["/home/mbp/src/nextest/fixtures/nextest-tests/target/debug/deps/cdylib_example-9222c584dde9dbd0", "--list", "--format", "terse"], exit_status: ExitStatus(unix_wait_status(32512)), stdout: [], stderr: [47, 104, 111, 109, 101, 47, 109, 98, 112, 47, 115, 114, 99, 47, 110, 101, 120, 116, 101, 115, 116, 47, 102, 105, 120, 116, 117, 114, 101, 115, 47, 110, 101, 120, 116, 101, 115, 116, 45, 116, 101, 115, 116, 115, 47, 116, 97, 114, 103, 101, 116, 47, 100, 101, 98, 117, 103, 47, 100, 101, 112, 115, 47, 99, 100, 121, 108, 105, 98, 95, 101, 120, 97, 109, 112, 108, 101, 45, 57, 50, 50, 50, 99, 53, 56, 52, 100, 100, 101, 57, 100, 98, 100, 48, 58, 32, 101, 114, 114, 111, 114, 32, 119, 104, 105, 108, 101, 32, 108, 111, 97, 100, 105, 110, 103, 32, 115, 104, 97, 114, 101, 100, 32, 108, 105, 98, 114, 97, 114, 105, 101, 115, 58, 32, 108, 105, 98, 116, 101, 115, 116, 45, 99, 98, 97, 98, 55, 49, 54, 57, 55, 100, 102, 57, 55, 98, 48, 48, 46, 115, 111, 58, 32, 99, 97, 110, 110, 111, 116, 32, 111, 112, 101, 110, 32, 115, 104, 97, 114, 101, 100, 32, 111, 98, 106, 101, 99, 116, 32, 102, 105, 108, 101, 58, 32, 78, 111, 32, 115, 117, 99, 104, 32, 102, 105, 108, 101, 32, 111, 114, 32, 100, 105, 114, 101, 99, 116, 111, 114, 121, 10] }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This is because unwrap prints the Debug form and the error holds a Vec<u8>.

There could be a few ways to change this but it seemed like the intention of the existing code was to keep the exact output in the error enum as bytes, and only do lossy conversion to UTF-8 from Display. So, I looked for a way to get the Display form printed, and it seemed like a good way was to just return the error, since the tests that call this already themselves return Results.

With this applied the failures are much more helpful:

error: creating test list failed

Caused by:
  for `cdylib-example`, command `/tmp/nextest-fixtureQWK72c/src/target/debug/deps/cdylib_example-9222c584dde9dbd0 --list --format terse` exited with code 127
--- stdout:

--- stderr:
/tmp/nextest-fixtureQWK72c/src/target/debug/deps/cdylib_example-9222c584dde9dbd0: error while loading shared libraries: libtest-cbab71697df97b00.so: cannot open shared object file: No such file or directory

---

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c44c6cd) 77.40% compared to head (bca344f) 77.39%.
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1228      +/-   ##
==========================================
- Coverage   77.40%   77.39%   -0.01%     
==========================================
  Files          72       72              
  Lines       17859    17859              
==========================================
- Hits        13823    13822       -1     
- Misses       4036     4037       +1     

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[sunshowers]

Thank you!