Rename DummyDateTime to MockDateTime

[zbraniecki]

This is a pure rename to get rid of the Dummy term.

I remember there were multiple options floating around. If you can settle on a different word than Mock, I'm happy to use it. If not, I suggest we land with Mock.

[coveralls]

Pull Request Test Coverage Report for Build 9c562dac626b52ef9097ac16d1532e476dedb0de-PR-301

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.169%

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Totals
Change from base Build 554b3b5669a54b7694ceea49730e71bef4c5998d:	0.0%
Covered Lines:	2886
Relevant Lines:	3692

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💛 - Coveralls