test: retry backup operations on failure because of pending operations

[olavloite]

Retry backup operations during tests that fail because too many other backup operations are pending at that moment.

Fixes #1019

[codecov]

Codecov Report

Merging #1092 (abb9ee3) into master (71d7fd1) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1092      +/-   ##
============================================
- Coverage     85.15%   85.12%   -0.03%     
+ Complexity     2720     2716       -4     
============================================
  Files           155      155              
  Lines         14351    14351              
  Branches       1358     1358              
============================================
- Hits          12221    12217       -4     
- Misses         1562     1566       +4     
  Partials        568      568              

Impacted Files	Coverage Δ	Complexity Δ
...ud/spanner/SessionPoolAsyncTransactionManager.java	85.03% <0.00%> (-1.58%)	13.00% <0.00%> (-2.00%)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.01% <0.00%> (-0.20%)	74.00% <0.00%> (-1.00%)
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <0.00%> (ø)	17.00% <0.00%> (-1.00%)

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