test(spanner): unflake ITBulkConnectionTest.testBulkCreateConnectionsMultiThreaded

[sakthivelmanii]

Summary

Follow-up to #14604 to address review feedback on ITBulkConnectionTest.testBulkCreateConnectionsMultiThreaded:

• Wrap the ExecutorService execution in a try-finally block calling executor.shutdownNow() and executor.awaitTermination(5L, TimeUnit.SECONDS) if the executor has not terminated when the test exits.
• Include a descriptive failure message in assertTrue(..., executor.awaitTermination(60L, TimeUnit.SECONDS)) and use JUnit assertTrue / assertNotNull assertions inside the worker task.

[gemini-code-assist]

Code Review

This pull request refactors the multithreaded connection test in ITBulkConnectionTest.java to ensure proper cleanup of the ExecutorService using a try-finally block, while also updating some assertions to standard JUnit style. The review feedback recommends adding a null check for the executor and properly handling the potential InterruptedException during awaitTermination in the finally block to avoid swallowing the exception.