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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