Adjust test_mount_filesystems to the latest blivet changes

The next version of blivet will switch from calling the 'mount'
command for mounting to libblockdev FS plugin (which use libmount)
so the test needs to be changed. This way the check will work with
both the old and the new blivet's code.

Related: storaged-project/blivet#1163

tests/unit_tests/pyanaconda_tests/modules/storage/test_module_storage.py

@@ -1548,29 +1548,26 @@ def test_activate_filesystems(self, patched_conf):
 
     @patch("os.chmod")
     @patch("os.makedirs")
-    @patch("blivet.util._run_program")
+    @patch("blivet.tasks.fsmount.FSMount.do_task")
     @patch("pyanaconda.core.util._run_program")
-    def test_mount_filesystems(self, core_run_program, blivet_run_program, makedirs, chmod):
+    def test_mount_filesystems(self, core_run_program, blivet_mount, makedirs, chmod):
         """Test MountFilesystemsTask."""
         storage = create_storage()
         storage._bootloader = Mock()
-        blivet_run_program.return_value = (0, "")
+        blivet_mount.return_value = None
         core_run_program.return_value = (0, "")
 
         task = MountFilesystemsTask(storage)
         task.run()
 
         # created mount points
-        makedirs.assert_any_call('/mnt/sysimage/dev', 0o755)
         makedirs.assert_any_call('/mnt/sysimage/tmp')
 
         # fixed permissions
         chmod.assert_any_call('/mnt/sysimage/tmp', 0o1777)
 
         # mounted devices
-        blivet_run_program.assert_any_call([
-            'mount', '-t', 'bind', '-o', 'bind,defaults', '/dev', '/mnt/sysimage/dev'
-        ])
+        blivet_mount.assert_any_call('/mnt/sysimage/dev', options='defaults')
 
         # remounted the root filesystem
         core_run_program.assert_any_call(