NetworkManager 1.15.2 wants keyfiles in /run to have the .nmconnection extension (LP: #1817655)

This appeared in NM upstream as 648c256b9014198aac388097e410999c68c4b452; and
previous versions of NetworkManager do not seem to care, so land the changes in
general, without changes for previous Ubuntu releases.

Adjust tests so they still work with this change.

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>