feat: Add the ability to disable test failures on report issue

[lukeredpath]

Adds the ability to disable the current default behaviour of triggering a test failure whenever reportIssue is called in a testing context with a new flag TestContext.emitsFailureOnReportIssue. This value can be overridden for a single operation using the withEmitsFailureOnReportIssue(_) helper function.

Resolves #145.

[adamjcampbell]

Awesome work, this is needed! I wonder if it'd be better if this behaviour was part of the existing IssueReporters array so that the existing method to disable things via:

withIssueReporters([]) {
  // Reported issues will be ignored here...
}

Would work as per the documentation

This would mean in a testing context IssueReporters.current should return an array including a TestReporter