Fix test_snapshot_list_param_marker when using predefined user

When using pre-provisoned credential, test_snapshot_list_param_marker
may fail occasionally, if there are other testcases running and if
other testcases also create snapshots.
This is to improve test_snapshot_list_param_marker to ensure it
succeed for the most time.

Change-Id: Ia21223b116a112254ec036e7a2ec03ecb9e2bb25
Closes-Bug: #1752228
1 file changed