tests: don't abort in fopen(/proc/mounts)

The mock fopen() function will abort if "/proc/mounts" is
requested with "r" permissions and VIR_CGROUP_MOCK_FILENAME
env var is not set.

Unfortunately this is triggering by the libselinux library
constructor when it tries to read /proc/mounts to find out
if selinuxfs is mounted in an unusual place.

This, however, only affects libselinux in Debian as that
opens with "r", while in Fedora / RHEL it opens "re" and
thus luckily never triggered the abort(), instead getting
an EACCESS.

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