Spanner: Increase timeouts to prevent test flakiness

[olavloite]

Fixes a possible flake in the InstanceAdminGaxTest. Example flake: https://source.cloud.google.com/results/invocations/de24a580-621b-4521-ba92-7bcc58fb2dbe/targets/github%2Fgoogle-cloud-java%2Fgoogle-cloud-clients%2Fgoogle-cloud-spanner%2Ftarget%2Fsurefire-reports/tests

[codecov]

Codecov Report

Merging #6155 into master will increase coverage by 0.36%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #6155 +/- ##
============================================
+ Coverage 47.01% 47.38% +0.36% - Complexity 25717 27182 +1465 
============================================
Files 2524 2524 Lines 269337 274612 +5275 Branches 31190 31383 +193 ============================================
+ Hits 126641 130125 +3484 - Misses 134018 134874 +856 - Partials 8678 9613 +935

Impacted Files	Coverage Δ	Complexity Δ
...va/com/google/cloud/compute/v1/InstanceClient.java	55.24% <0%> (-7.81%)	143% <0%> (+35%)
...onitoring/v3/NotificationChannelServiceClient.java	64.49% <0%> (-7.13%)	43% <0%> (+10%)
...ava/com/google/cloud/compute/v1/ProjectClient.java	57.07% <0%> (-6.99%)	55% <0%> (+13%)
.../com/google/cloud/compute/v1/TargetPoolClient.java	57.73% <0%> (-6.98%)	47% <0%> (+11%)
...va/com/google/cloud/compute/v1/SnapshotClient.java	55.73% <0%> (-6.88%)	31% <0%> (+7%)
...oogle/cloud/kms/v1/KeyManagementServiceClient.java	62.88% <0%> (-6.84%)	103% <0%> (+25%)
...com/google/cloud/compute/v1/ReservationClient.java	59.14% <0%> (-6.83%)	39% <0%> (+9%)
...a/com/google/cloud/compute/v1/NodeGroupClient.java	60.36% <0%> (-6.82%)	51% <0%> (+12%)
.../com/google/cloud/compute/v1/SubnetworkClient.java	60.36% <0%> (-6.82%)	51% <0%> (+12%)
...d/compute/v1/RegionInstanceGroupManagerClient.java	54.5% <0%> (-6.79%)	51% <0%> (+12%)
... and 639 more

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