Make mocked_s3utils_with_sse available in ff_mocks.py

[netsettler]

I want to be able to use some of this better mocking tech in other repos, so I moved it to a place where I could access it.

[coveralls]

Pull Request Test Coverage Report for Build 4294864518

• 16 of 16 (100.0%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 73.825%

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Files with Coverage Reduction	New Missed Lines	%
dcicutils/ff_utils.py	2	70.63%
<!--	Total:	2		-->

Totals
Change from base Build 4243873727:	0.04%
Covered Lines:	5971
Relevant Lines:	8088

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💛 - Coveralls