libqtest: Inline g_assert_no_errno()

Macro g_assert_no_errno() intrudes into GLib's namespace.  It's also
pretty pointless.  Inline.

At one call site, its redundancy is now obvious.  Delete it there.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180926122933.3858-1-armbru@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>