tests: fix e1000e leaks

Spotted by ASAN.

This hunk adds an assertion. It checks that we're finding no more than
one e1000e device: each hit allocates, but there is only one g_free().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
