test: close Spanner instance after test

[olavloite]

The SpannerGapicRpcTest.testCustomUserAgent test opened a new Spanner instance that was not closed after the test had finished. This would cause the threads that were used by that instance to remain active until the entire test run had finished. If this test happened to be executed before one of the tests that verify that all threads are closed when a Spanner instance is closed, the test would fail as it would detect the threads that were still active from the other test case.

Fixes #889

[codecov]

Codecov Report

Merging #894 (df306b3) into master (b2b7bb2) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #894   +/-   ##
=========================================
  Coverage     85.17%   85.17%           
- Complexity     2619     2621    +2     
=========================================
  Files           145      145           
  Lines         14262    14262           
  Branches       1378     1378           
=========================================
+ Hits          12147    12148    +1     
+ Misses         1540     1538    -2     
- Partials        575      576    +1     

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/spanner/SessionPool.java	89.03% <0.00%> (ø)	72.00% <0.00%> (ø%)
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <0.00%> (ø)	18.00% <0.00%> (+1.00%)
.../google/cloud/spanner/AbstractLazyInitializer.java	100.00% <0.00%> (+7.14%)	5.00% <0.00%> (+1.00%)

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