Adapt number of credentials tests executed based on system performance

Faster systems can run more tests in the same amount of time, so allow them
to run more tests, while the slower systems run fewer tests. Should increase
the chances that each authentication test is run at least once in the 
multi-configuration job.
Merge commit '14bc4f381a2299aa7052a4d396185c5d8ab4b87b' into HEAD