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

[wneumann]

Added two tests that look for corner cases that I have had students trip up on. Single nucleotide strands, and strands that contain both valid and invalid nucleotides (current validation test was only with all invalid characters).

I also modified the tests to remove force unwrapping and to use XCTAsserNonNil) instead (tried to use XCTUnwrap, but found out the infrastructure doesn't yet support XCTUnwrap). As mentors, we would encourage our students to not use force unwrapping in their code, and we shouldn't use it in ours.