spanner-jdbc: Add tests for JdbcStatement

[olavloite]

Adds additional tests for JdbcStatement. Also fixes an issue that executing statements and requesting generated keys to be returned should throw a SQLFeatureNotSupportedException.

[codecov]

Codecov Report

Merging #6158 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #6158 +/- ##
============================================
+ Coverage 47.38% 47.39% +<.01% - Complexity 27182 27192 +10 
============================================
Files 2524 2524 Lines 274612 274614 +2 Branches 31383 31385 +2 ============================================
+ Hits 130125 130150 +25 + Misses 134874 134851 -23 
Partials 9613 9613

Impacted Files	Coverage Δ	Complexity Δ
.../com/google/cloud/spanner/jdbc/JdbcConnection.java	30.37% <ø> (ø)	17 <0> (ø)	⬇️
...a/com/google/cloud/spanner/jdbc/JdbcStatement.java	67.74% <100%> (+16.1%)	38 <0> (+10)	⬆️
...ain/java/com/google/cloud/pubsub/v1/Publisher.java	89.11% <0%> (-0.35%)	40% <0%> (ø)

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