[nextest-runner] fix underflow in delay computation

[sunshowers]

Fixes #2770.

[codecov]

Codecov Report

❌ Patch coverage is 98.70130% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.89%. Comparing base (217e109) to head (2d837a8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
nextest-runner/src/reporter/displayer/progress.rs	98.70%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2771      +/-   ##
==========================================
+ Coverage   79.56%   79.89%   +0.33%     
==========================================
  Files         112      112              
  Lines       25726    25799      +73     
==========================================
+ Hits        20469    20613     +144     
+ Misses       5257     5186      -71     

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[trunk-io]

     

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