spanner-tests: Sleep uninterruptedly to prevent test failures

[olavloite]

Prevent a spurious wakeup or thread interrupt from causing a test failure due to an unexpected execution time.

Updates #6075

[codecov]

Codecov Report

Merging #6096 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #6096      +/-   ##
============================================
- Coverage     47.38%   47.38%   -0.01%     
- Complexity    27182    27195      +13     
============================================
  Files          2523     2523              
  Lines        274581   274581              
  Branches      31380    31375       -5     
============================================
- Hits         130123   130113      -10     
- Misses       134850   134858       +8     
- Partials       9608     9610       +2

Impacted Files	Coverage Δ	Complexity Δ
.../java/com/google/cloud/testing/CommandWrapper.java	87.87% <0%> (-9.1%)	13% <0%> (ø)
.../cloud/datastore/testing/LocalDatastoreHelper.java	80.59% <0%> (-4.48%)	17% <0%> (ø)
...able/gaxx/reframing/ReframingResponseObserver.java	88.99% <0%> (-1.84%)	29% <0%> (-1%)
...oogle/cloud/spanner/jdbc/SingleUseTransaction.java	86.87% <0%> (-0.91%)	39% <0%> (-1%)

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