test: allow coverage threshold to be enforced

If COV_ENFORCE_THRESHOLD is set, tests run in coverage mode will
exit with an error, if line coverage is below the percentage
threshold.

PR-URL: https://github.com/nodejs/node/pull/25675


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>