tests: Use dummy modulesdir for DNF

Otherwise it tries to ensure it exists, and since the default is
/etc/dnf/modules.d, it's causing problems if the directory does not
exist and user does not have permissions to create it.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/test_gather.py

@@ -1810,6 +1810,7 @@ class DNFDepsolvingTestCase(DepsolvingBase, unittest.TestCase):
         conf = Conf(base_arch)
         conf.persistdir = persistdir
         conf.cachedir = self.cachedir
+        conf.modulesdir = os.path.join(persistdir, 'modules.d')
         if exclude:
             conf.exclude = exclude
         dnf = DnfWrapper(conf)