Improve test coverage

[thomasjo]

This is a tracking issue to help us improve the overall test coverage.

Some functionality currently has little to no spec coverage, and this is something that needs to be addressed. The list below is updated as we discover (and resolve) functionality which needs better coverage.

• TexifyBuilder has zero coverage, which must be improved.
• KnitrBuilder has OK coverage, but should be improved.
  • canProcess.
• LatexmkBuilder has OK coverage, but should be improved.
  • canProcess.
  • logStatusCode.
  • All permutations supported by constructArgs.
• DefaultLogger has zero coverage, which must be improved.
• LogParser has reasonable coverage, but can be improved.
• BuilderRegistry has poor coverage, which must be improved.
  • getAllBuilders.
  • All permutations of resolveAmbiguousBuilders.
• Composer has less than ideal coverage, which should be improved.
  • Try to test sync — might need too much mocking...
  • moveResult.
  • resolveOutputFilePath.
  • showResult
  • Status bar indicator functionality.
• Latex has OK coverage, but the tests should be improved.

[thomasjo]

This is being resolved in #224

[yitzchak]

Resolved by #301 pending fresh review.