Improved IntegratedFixture and static check cleanups (C4-932)

[netsettler]

• Show less uninteresting traceback info for static tests.
• Small incompatible change to recently released qa_checkers.confirm_no_uses to remove the if_used argument in favor of a simpler implementation. Since this is testing-only, not something used in production, and since there are believed to be no uses outside the repo, no major version bump.
• Make class initialization of IntegratedFixture happen at instance instantiation time. That simplifies the loading actions needed. Those can happen in conftest.py rather than in ff_mocks, which should allow dcicutils.ff_mocks to be imported more easily (C4-932).

[coveralls]

Pull Request Test Coverage Report for Build 3161551459

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 90 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.05%) to 71.365%

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Files with Coverage Reduction	New Missed Lines	%
dcicutils/qa_checkers.py	37	69.65%
dcicutils/ff_mocks.py	53	62.24%
<!--	Total:	90		-->

Totals
Change from base Build 3146164003:	0.05%
Covered Lines:	5570
Relevant Lines:	7805

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💛 - Coveralls