expect_success() checks that there's exactly one success and no failures;
expect_failure() checks that there's exactly one failure and no successes.
expect_snapshot_failure() records the failure message so that you can
manually check that it is informative.
Use show_failure() in examples to print the failure message without
throwing an error.
expect_success(expr)expect_failure(expr, message = NULL, ...)
expect_snapshot_failure(expr)
show_failure(expr)
Code to evaluate
Check that the failure message matches this regexp.
Other arguments passed on to expect_match().