Add unstable tag to test with intermittent failures

The test 'test_create_extend_and_write' started failing
intermittently when run under CIFS protocol. This change adds
a unstable test tag to the test to the result will be ignored
when it fails again.

Closes-Bug: #1903922
Change-Id: Ic5527243c0b08dbbfbc849a7c2e79ff927c23098
diff --git a/manila_tempest_tests/tests/scenario/test_share_extend.py b/manila_tempest_tests/tests/scenario/test_share_extend.py
index d6a23d0..595ddd4 100644
--- a/manila_tempest_tests/tests/scenario/test_share_extend.py
+++ b/manila_tempest_tests/tests/scenario/test_share_extend.py
@@ -150,7 +150,12 @@
 
 
 class TestShareExtendCIFS(manager.BaseShareScenarioCIFSTest, ShareExtendBase):
-    pass
+
+    @decorators.idempotent_id('4a9f5cf9-990d-4a35-9ac7-87ef593a09e3')
+    @decorators.unstable_test(bug='1903922')
+    @tc.attr(base.TAG_POSITIVE, base.TAG_BACKEND)
+    def test_create_extend_and_write(self):
+        super(TestShareExtendCIFS, self).test_create_extend_and_write()
 
 
 class TestBaseShareExtendScenarioCEPHFS(manager.BaseShareScenarioCEPHFSTest,