Mock leaked _execute() calls in driver tests

Some drivers have _execute() calls being linked, causing tests to prompt
for password and eventual fail for time out. This prevents those
_execute calls from happening.

Closes-Bug: #1404414
Change-Id: I43a11c065799efe6f5b81dc15e1f7232c80d1070
Merge "Mock leaked _execute() calls in driver tests"