"Some tests to cover hash_index"

I am attaching the patch to improve some coverage of hash index code [1].
I have added some basic tests, which mainly covers overflow pages. It took 2 sec extra time in my machine in parallel schedule.

		Hit	Total	Coverage
old tests	Line Coverage	780	1478	52.7
	Function Coverage	63	85	74.1
improvement after tests	Line Coverage	1181	1478	79.9 %
	Function Coverage	78	85	91.8 %

[1]Concurrent Hash Index.
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