spanner-jdbc: Add tests for JdbcConnection

[olavloite]

Adds additional tests for JdbcConnection. Also fixes a small issue in JdbcConnection that caused the method isValid(int) to throw an exception when called on a closed connection, instead of returning false as it should have.

[kolea2]

LGTM just needs a rebase

[codecov]

Codecov Report

Merging #6157 into master will increase coverage by 0.05%.
The diff coverage is 75%.

@@ Coverage Diff @@## master #6157 +/- ##
============================================
+ Coverage 47.41% 47.47% +0.05% - Complexity 27258 27402 +144 
============================================
Files 2524 2524 Lines 274663 274664 +1 Branches 31414 31414 ============================================
+ Hits 130239 130402 +163 + Misses 134783 134621 -162 
Partials 9641 9641

Impacted Files	Coverage Δ	Complexity Δ
.../com/google/cloud/spanner/jdbc/JdbcConnection.java	56.96% <75%> (+26.58%)	41 <0> (+24)	⬆️
...ain/java/com/google/cloud/pubsub/v1/Publisher.java	89.11% <0%> (-0.35%)	40% <0%> (ø)
...gle/cloud/spanner/jdbc/AbstractJdbcConnection.java	65.27% <0%> (+1.38%)	21% <0%> (+1%)	⬆️
...ogle/cloud/spanner/jdbc/AbstractJdbcStatement.java	55.26% <0%> (+5.26%)	29% <0%> (+5%)	⬆️
...ogle/cloud/spanner/jdbc/AbstractJdbcResultSet.java	100% <0%> (+95.79%)	117% <0%> (+114%)	⬆️

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