Backport: add granularity_sleep, fixes #9150 (from #9151) to 1.4-maint

[ThomasWaldmann]

This is a backport of PR #9151 to the 1.4-maint branch.

Original PR: #9151
Original commit: 199f708

Notes for backport:

• The original PR touched tests under testsuite/archiver which do not exist on 1.4-maint; those changes were intentionally omitted to keep deletions on 1.4-maint.
• Integrated the new helper testsuite/__init__.py: granularity_sleep() with platform-specific timings (Darwin: 1.0s, Windows: 0.2s or 15s for ctime_quirk, others: 0.02s).

Please review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.57%. Comparing base (a25856a) to head (e06935f).
⚠️ Report is 5 commits behind head on 1.4-maint.

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #9190      +/-   ##
=============================================
- Coverage      80.59%   80.57%   -0.02%     
=============================================
  Files             38       38              
  Lines          11251    11251              
  Branches        1769     1769              
=============================================
- Hits            9068     9066       -2     
- Misses          1614     1615       +1     
- Partials         569      570       +1     

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