Let test suites define completion criterias

[solsson]

No description provided.

[solsson]

This PR basically adds the following new metrics and leaves the rest for workflows, like Yolean/ystack#30:

# HELP assert_files_seen Unique spec file paths that have been seen in onTestResult
# TYPE assert_files_seen gauge
assert_files_seen 2

# HELP assert_completions_passed Gets a value if a spec names assert-completion.spec.js has run
# TYPE assert_completions_passed gauge
assert_completions_passed 5

# HELP assert_completions_remaining The number of failures for assert-completion.spec.js (pending and todo are ignored)
# TYPE assert_completions_remaining gauge
assert_completions_remaining 0

The assumption is that workflows can use assert_completions_remaining existence AND ==0, probably matching label namespace or pod, as a criteria to terminate specs runners.