mach tests command is broken in multiple ways. This mostly stems from changes to the test shell scripts that are bad.
mach tests gtests fails because the PRNG test fails (cause is probably somewhere in bug 1496124).
But worse probably is that parsing of results is not working properly.
results.html as well as the command line overview claim that tests are passed when they actually failed. This is easy to reproduce by breaking any gtest.
I also see things like
gtests.sh: #14: freebl_gtest run successfully - Core file is detected - FAILED in CI logs  such that I have doubts that green on CI actually means everything is ok.
Marcus can you have a look at this as you're looking at the test scripts already?