As part of simplifying the CI setup I plan to not explicitly specify
`--jobs=4` to `tap` going forward. Upgrading to a newer version of `tap`
means that it will automatically parallelize jobs according to available
CPU count, which should be better anyway. Only one of our tests was
incompatible with newer versions of `tap`, so this commit includes a
compatibility fix there.
Also, by default newer versions of `tap` calculate coverage and consider
the test run to fail if below the configured thresholds. The default is
100% coverage and we're not there, so I adjusted the thresholds to match
where we're at for now. We can ratchet those up over time.
