[nextest-runner] improve error handling for child process management

[sunshowers]

• Don't eat up stdout/stderr if there's an error that occurs after the process is spawned
• Scripts can now combine stdout and stderr (though this isn't exposed in the UI yet).
• Fix up names to reflect that "child" can mean either a test process or a script process.

This is going to need a bunch of TLC, but I think this is generally a good shape for the types to be in.

[codecov]

Codecov Report

Attention: Patch coverage is 66.80498% with 80 lines in your changes missing coverage. Please review.

Project coverage is 80.00%. Comparing base (6bd1209) to head (f429e8d).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
nextest-runner/src/runner.rs	61.01%	23 Missing ⚠️
nextest-runner/src/reporter/structured/libtest.rs	65.07%	22 Missing ⚠️
nextest-runner/src/reporter/displayer.rs	74.46%	12 Missing ⚠️
nextest-runner/src/reporter/aggregator.rs	0.00%	11 Missing ⚠️
nextest-runner/src/config/scripts.rs	60.00%	6 Missing ⚠️
nextest-runner/src/errors.rs	89.74%	4 Missing ⚠️
nextest-runner/src/test_command/imp.rs	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1930      +/-   ##
==========================================
+ Coverage   79.82%   80.00%   +0.17%     
==========================================
  Files          81       81              
  Lines       21048    21086      +38     
==========================================
+ Hits        16802    16869      +67     
+ Misses       4246     4217      -29     

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