AMPS-1252: Use mock, not jira-core's IssueImpl

Disclaimer: I think I got the right stakeholders, but I'm not 100% sure that Tom and Lucas are correct because your names were not a perfect match for what Confluence lists. Apologies if I've gotten them wrong.

See https://answers.atlassian.com/questions/15840218 for what prompted this. The short version is that the unit tests that AMPS creates for workflow functions are trying to use jira-core's IssueImpl class directly instead of mocking MutableIssue like a good unit test should.

I cleaned up the formatting quite a bit here, too (spaces between operators and the like).

The CI is here and green.