tests: skip systemd activation test if FDs 3/4/5 are open

With systemd activation the passed in file descriptors are required to
be numbered from STDERR_FILENO + 1 onwards. The unit tests thus require
FDs 3, 4 and 5 to be available.

This may not be the case in all environments in which the tests run. For
example on RHEL7 it was seen that a library constructor (gcrypt probably)
opens /dev/urandom and leaves the file handle open. This means FD 3 is
not available and the activation tests fail.

The best way to deal with this would be to create a standalone helper
program for the tests, but that's much more work than just skipping the
tests if we notice we have the problem.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>