highlight interesting parts of the output

[sunshowers]

With very long outputs, it can often be hard to tell which parts correspond immediately to the failure. Highlight those parts so users can easily tell.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 55.83333% with 265 lines in your changes missing coverage. Please review.

Project coverage is 79.06%. Comparing base (a9bfefb) to head (d09fcaa). Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
cargo-nextest/src/dispatch.rs	0.00%	102 Missing :warning:
nextest-runner/src/reporter/helpers.rs	80.36%	64 Missing :warning:
nextest-runner/src/reporter/displayer.rs	48.35%	47 Missing :warning:
nextest-runner/src/test_output.rs	41.02%	23 Missing :warning:
nextest-runner/src/reporter/aggregator.rs	0.00%	16 Missing :warning:
cargo-nextest/src/errors.rs	0.00%	10 Missing :warning:
nextest-runner/src/reporter/structured/libtest.rs	85.71%	2 Missing :warning:
nextest-runner/src/runner.rs	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1678 +/- ## ========================================== - Coverage 79.71% 79.06% -0.66% ========================================== Files 78 79 +1 Lines 19339 19726 +387 ========================================== + Hits 15417 15597 +180 - Misses 3922 4129 +207 ```

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