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: NDaniel P. Berrangé <berrange@redhat.com>

tests/vircgroupmock.c

@@ -460,8 +460,10 @@ FILE *fopen(const char *path, const char *mode)
     }
 
     if (type) {
-        if (!filename)
-            abort();
+        if (!filename) {
+            errno = EACCES;
+            return NULL;
+        }
         if (virAsprintfQuiet(&filepath, "%s/vircgroupdata/%s.%s",
                              abs_srcdir, filename, type) < 0) {
             abort();