tests: Fix ERP test cases to close separate AS (ifname=as-erp)

tests: Fix ERP test cases to close separate AS (ifname=as-erp)

The earlier change from using apdev[1] to using a no-interface
ifname=as-erp in the ERP related test cases ended up leaving the
separate authentication server interface running after the end of the
test case. This left UDP port 18128 open and any consecutive test case
needing this port failed to start another authentication server.

This can be reproduced with following test case sequence:
fils_auth_gtk_rekey authsrv_unknown_user

Fixes: e374def207c1 ("tests: Start ERP authentication server without AP")
Signed-off-by: Jouni Malinen <j@w1.fi>