test: Add exponential back-off to rm retry

[topolarity]

#1100 was not enough, so let's try much longer delays w/ exponential back-off (20 retries, 100s total in the worst case).

If this doesn't work, we may have to re-structure the file management in the tests to avoid this issue altogether.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.45%. Comparing base (45920de) to head (d1e2245).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1102   +/-   ##
=======================================
  Coverage   85.45%   85.45%           
=======================================
  Files           3        3           
  Lines         832      832           
=======================================
  Hits          711      711           
  Misses        121      121           

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