Changes

s1gw: TC_e_rab_setup: complete E-RAB release

The MME originated E-RAB RELEASE procedure includes both:

* [ENB <- MME] E-RAB RELEASE COMMAND, and
* [ENB -> MME] E-RAB RELEASE RESPONSE.

The later was overlooked in a99224c9, so add it.

Change-Id: I856248d825b6ecf0635590b7bf02593cfae893d3
Fixes: a99224c9 "s1gw: TC_e_rab_setup: also test E-RAB release"