wneumann: add and improve tests for swift/nucleotide-count#390

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wneumann wants to merge 3 commits intoexercism:mainfrom wneumann:wneumann/add-and-improve-tests

+46-8

Commits on May 10, 2020

Added two test that I have had students trip up on. Single nucleotide strands, and strands that contain valid and
wneumann
committed
Cleaned up spurious trailing whitespace
wneumann
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Replaced XCode only XCTUnwrap with XCTAssertNotNil and a guard let.
wneumann
committed