[nextest-runner] make delays obey SIGTSTP and SIGCONT

[sunshowers]

Make delay timers obey these signals.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 80.12%. Comparing base (b55dde6) to head (efd2e92).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
nextest-runner/src/runner.rs	60.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1937      +/-   ##
==========================================
- Coverage   80.13%   80.12%   -0.02%     
==========================================
  Files          82       82              
  Lines       21171    21194      +23     
==========================================
+ Hits        16966    16982      +16     
- Misses       4205     4212       +7     

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