Skip all Autotest parallelism tests if using Guix’s bash.

Skip all Autotest parallelism tests if using Guix’s bash.

It is not clear to me why, but the “parallel autotest and signal
handling” test malfunctions if the active shell is Guix bash.  I don’t
think it’s worth investigating in detail, considering I intend to
reimplement parallel autotest using the same technique that Automake’s
parallel test driver uses, i.e. make -j, which should eliminate this
entire class of problems.