Spanner: Simplify and unflake SpannerGaxRetryTest

[olavloite]

Fixes the flake from this build: https://source.cloud.google.com/results/invocations/e06a185f-0e72-4e5d-a914-29944231d7af/targets/cloud-devrel%2Fclient-libraries%2Fjava%2Fgoogle-cloud-java%2Fpresubmit%2Fjava8-win/log

Also cleans up, speeds up and simplifies the test case and removes some unused code.

[codecov]

Codecov Report

Merging #6163 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #6163 +/- ##
============================================
+ Coverage 47.45% 47.49% +0.03% - Complexity 27372 27432 +60 
============================================
Files 2524 2524 Lines 274664 274672 +8 Branches 31414 31414 ============================================
+ Hits 130347 130445 +98 + Misses 134676 134594 -82 + Partials 9641 9633 -8

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/com/google/cloud/storage/Storage.java	79.72% <0%> (ø)	0% <0%> (ø)	⬇️
...le/cloud/compute/v1/ProjectGlobalSnapshotName.java	56.36% <0%> (ø)	8% <0%> (ø)	⬇️
...cloud/compute/v1/ProjectZoneInstanceGroupName.java	57.14% <0%> (ø)	8% <0%> (ø)	⬇️
...1/ProjectZoneNetworkEndpointGroupResourceName.java	57.14% <0%> (ø)	8% <0%> (ø)	⬇️
...ompute/v1/ProjectRegionSubnetworkResourceName.java	57.14% <0%> (ø)	8% <0%> (ø)	⬇️
...ud/compute/v1/ProjectGlobalSecurityPolicyName.java	56.36% <0%> (ø)	8% <0%> (ø)	⬇️
...ud/compute/v1/ProjectGlobalTargetTcpProxyName.java	56.36% <0%> (ø)	8% <0%> (ø)	⬇️
...oud/compute/v1/ProjectGlobalBackendBucketName.java	56.36% <0%> (ø)	8% <0%> (ø)	⬇️
.../cloud/compute/v1/ProjectRegionTargetPoolName.java	57.14% <0%> (ø)	8% <0%> (ø)	⬇️
...d/compute/v1/ProjectGlobalTargetHttpProxyName.java	56.36% <0%> (ø)	8% <0%> (ø)	⬇️
... and 79 more

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