Fix misuse of assert_has_calls
Fix misuse of assert_has_calls

The 'assert_has_calls' is a method, not a variable.

Change-Id: I6aea978b8fc9adf4a062c149c84d6562f5adec04
Closes-Bug: 1989280
Signed-off-by: Takashi Natsume <2c48eb5bf73c355a2d5583bc776dffa383cdf819@gmail.com>

openstack/oslo.db,openstack/oslo.db