fix outcome label in goss_tests_run_outcomes_total

[sebhoss]

The prometheus metric goss_tests_run_outcomes_total has a label called 'outcome' which suffers from two problems:

1. It won't be set to 'pass' in case all tests succeed
2. The order of test results determines the final value of the label, e.g. if there is a failed & skipped test then the outcome will be skipped, but if order of results is reversed the outcome will become failed.

This commit fixes both by introducing a helper function which determines whether the outcome can be changed and adds tests to verify that behavior.

fixes #789

Checklist

• [x] make test-all (UNIX) passes. CI will also test this
• [x] unit and/or integration tests are included (if applicable)
• [ ] documentation is changed or added (if applicable)

Description of change

[sebhoss]

pinging @petemounce @timeu since you guys were involved in the original implementation

[timeu]

From my point of view it looks good. Thanks for the fix

[aelsabbahy]

Merging based on @timeu review.

Thank you for the great contribution @sebhoss

[sebhoss]

Thanks everyone!