test: improve execution speed of tests

[olavloite]

• Improves overall test execution time (locally) with approximately 10 seconds.
• Updates more try-fail-catch to assertThrows

[olavloite]

The changes in this file look very big (on GitHub), but are in reality quite small:

1. An additional package-private constructor for test purposes has been added that allows creating a GapicSpannerRpc without creating and initializing the gRPC stubs. This allows testing methods that do not communicate with any backend without the need to setup a mock server, and without the additional execution time of initializing and closing the stubs.
2. The initialize and close methods contain an if-statement to check whether to actually initialize / close stubs.

[codecov]

Codecov Report

Merging #1151 (0340e2f) into master (cd45643) will increase coverage by 0.03%.
The diff coverage is 83.70%.

❗ Current head 0340e2f differs from pull request most recent head f2da9b1. Consider uploading reports for the commit f2da9b1 to get more accurate results

@@             Coverage Diff              @@
##             master    #1151      +/-   ##
============================================
+ Coverage     84.86%   84.89%   +0.03%     
- Complexity     2762     2769       +7     
============================================
  Files           156      156              
  Lines         14318    14331      +13     
  Branches       1377     1379       +2     
============================================
+ Hits          12151    12167      +16     
+ Misses         1596     1593       -3     
  Partials        571      571              

Impacted Files	Coverage Δ	Complexity Δ
...m/google/cloud/spanner/spi/v1/GapicSpannerRpc.java	80.77% <83.70%> (+0.57%)	100.00 <1.00> (+3.00)
...le/cloud/spanner/connection/StatementExecutor.java	75.00% <0.00%> (-1.93%)	8.00% <0.00%> (ø%)
...a/com/google/cloud/spanner/SessionPoolOptions.java	67.96% <0.00%> (-1.57%)	18.00% <0.00%> (+1.00%)	⬇️
...ain/java/com/google/cloud/spanner/SessionPool.java	88.55% <0.00%> (ø)	82.00% <0.00%> (+1.00%)
...cloud/spanner/connection/ReadWriteTransaction.java	82.09% <0.00%> (+0.27%)	73.00% <0.00%> (+1.00%)
...gle/cloud/spanner/AsyncTransactionManagerImpl.java	75.71% <0.00%> (+1.42%)	16.00% <0.00%> (+1.00%)
.../cloud/spanner/spi/v1/SpannerErrorInterceptor.java	58.82% <0.00%> (+5.88%)	3.00% <0.00%> (ø%)

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