[POOL-419] Add failing tests for POOL-419.

[rajucomp]

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[rajucomp]

Hi ggregory. Hope you are well. I really thought about your comment last year to focus on bug fixes. I have tried my best.

I have created a PR at #385.

1. The PR adds a test to demonstrate that the value of getMaxActive can be a negative number.
2. The PR also proposes a solution to fix this issue.

During my debugging, I found that when the returned objects were being added back to the set of the idle objects, there was a possibility where they could have been invalidated by another thread leading to invalid objects being added back to the pool which is not desirable.

I am eager to know you critical thoughts on this. Thanks!

[rajucomp]

@garydgregory Requesting a review. Thanks!