spanner-jdbc: Unflake ITTransactionRetryTest

[olavloite]

The exact number of times a transaction aborted cannot be be predicted, only the minimum number of aborts.

Fixes flake https://source.cloud.google.com/results/invocations/014c5a4b-cc16-4795-a707-f15f2b3a8029/targets/github%2Fgoogle-cloud-java%2Fgoogle-cloud-clients%2Fgoogle-cloud-contrib%2Fgoogle-cloud-spanner-jdbc%2Ftarget%2Ffailsafe-reports/tests;group=com.google.cloud.spanner.jdbc.it.ITTransactionRetryTest%28sponge_log%29;test=testAbortWithUnseenConcurrentInsert;row=1

[codecov]

Codecov Report

Merging #6159 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #6159 +/- ##
============================================
+ Coverage 47.38% 47.38% +<.01% - Complexity 27182 27183 +1 
============================================
Files 2524 2524 Lines 274612 274612 Branches 31383 31383 ============================================
+ Hits 130125 130126 +1 
Misses 134874 134874 + Partials 9613 9612 -1

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/pubsub/v1/Publisher.java	89.11% <0%> (-0.35%)	40% <0%> (ø)
...oogle/cloud/spanner/jdbc/SingleUseTransaction.java	87.72% <0%> (+0.9%)	40% <0%> (+1%)	⬆️

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