[mozmill_endurance_test] Revert removal of modal-dialog, fix modal-dialog bug instead

Was not paying enough attention, other modules use this and the upstream
fix is a simple null check, so just backport that.

This reverts commit ad7738d34438991a42f475b08be6bed8330d45e0.
[mozmill_endurance_test/slimtest_config] Add settleWaitTime as a config var instead of hardcoding it