test: fail if the same token is seen twice

[olavloite]

The backup pagination test should fail if the same page token is returned twice by the backend, instead of fetching the same page indefinitely. Further investigation is necessary if that actually happens, as it is not something that is expected.

[codecov]

Codecov Report

Merging #983 (c1add12) into master (fc305b7) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #983      +/-   ##
============================================
- Coverage     85.11%   85.09%   -0.02%     
+ Complexity     2619     2616       -3     
============================================
  Files           154      154              
  Lines         14319    14319              
  Branches       1331     1331              
============================================
- Hits          12187    12185       -2     
- Misses         1567     1570       +3     
+ Partials        565      564       -1     

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/spanner/SessionPool.java	89.12% <0.00%> (-0.20%)	72.00% <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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