spanner-jdbc: Use NoCredentials for tests

[olavloite]

Use NoCredentials in test cases where that is possible. For JDBC connections it is not possible to explicitly specify NoCredentials. Instead, the JDBC driver should default to NoCredentials when the user has specified a plain text connection and not specified any specific credentials file.

Fixes#6075

[codecov]

Codecov Report

Merging #6145 into master will decrease coverage by 0.28%.
The diff coverage is 83.33%.

@@ Coverage Diff @@## master #6145 +/- ##
============================================
- Coverage 47.38% 47.1% -0.29% + Complexity 27198 27183 -15 
============================================
Files 2523 2523 Lines 274600 274462 -138 Branches 31378 30561 -817 ============================================
- Hits 130127 129277 -850 + Misses 134861 134782 -79 - Partials 9612 10403 +791

Impacted Files	Coverage Δ	Complexity Δ
...m/google/cloud/spanner/jdbc/ConnectionOptions.java	82.2% <83.33%> (-0.62%)	46 <0> (+2)
...va/com/google/cloud/compute/v1/InstanceClient.java	48.8% <0%> (-6.45%)	143% <0%> (ø)
.../com/google/cloud/compute/v1/RegionDiskClient.java	47.9% <0%> (-5.99%)	43% <0%> (ø)
...ava/com/google/cloud/compute/v1/ProjectClient.java	51.14% <0%> (-5.94%)	55% <0%> (ø)
...ava/com/google/cloud/compute/v1/NetworkClient.java	49.63% <0%> (-5.84%)	35% <0%> (ø)
...ava/com/google/cloud/compute/v1/LicenseClient.java	50.41% <0%> (-5.79%)	31% <0%> (ø)
...va/com/google/cloud/compute/v1/SnapshotClient.java	50% <0%> (-5.74%)	31% <0%> (ø)
.../java/com/google/cloud/compute/v1/ImageClient.java	49.04% <0%> (-5.74%)	43% <0%> (ø)
.../google/cloud/compute/v1/TargetSslProxyClient.java	48.57% <0%> (-5.72%)	35% <0%> (ø)
.../google/cloud/compute/v1/SecurityPolicyClient.java	48.1% <0%> (-5.7%)	39% <0%> (ø)
... and 179 more

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