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
1 file changed