GEODE-5574: unit test coverage for RegionMapDestroy

[dschneider-pivotal]

This is a major rewrite of RegionMapDestroyTest. The test was changed enough that I doubt the diffs will be helpful. I changed it to use mocks except for the class under test.
Each test method now has the pattern "given, do, verify".
This unit test now covers 90% of the class (used to be 60%) and the uncovered code is debug logging and what I think is dead code.

I made one small change to the product to use "putEntryIfAbsent" instead of "getEntryMap().putIfAbsent".

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[dschneider-pivotal]

the integrationTest failure was a timeout and I see that other ci runs also had this issue. Since all this PR does is modify a unit test, I'm not going to hold it up the integrationTest timeout.