TEST: Ignore atime in test_filemanip.test_recopy

[effigies]

From #1617, comparing stat on an unchanged file can falsely indicate a change if the filesystem sets atimes.

The first commit uses a helper function to cause tests to always span a clock tick, to demonstrate that all test-timing-related bugs (e.g. #1586) are handled, so we don't have to keep coming back to this.

The second commit removes the delays so that the tests run in <1s instead of ~13s.

Given that this follows on #1391, #1570 and #1586, no changelog entry.

[codecov-io]

Current coverage is 70.88% (diff: 100%)

Merging #1634 into master will decrease coverage by <.01%

@@             master      #1634   diff @@
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  Files          1019       1019          
  Lines         51277      51279     +2   
  Methods           0          0          
  Messages          0          0          
  Branches       7265       7265          
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- Hits          36351      36348     -3   
- Misses        13840      13843     +3   
- Partials       1086       1088     +2   

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