Replace `self.stop` with `Misc_Helpers.f_shutdown()`, and add it after existing `setverdict(fail, ...)` calls that previously fell through.
In all these cases there is no meaningful way to continue: the caller of these functions usually expects a valid value to be returned, and continuing (not stopping) oftentimes results in confusing DTEs.
s1gw: f_REST_*(): use tr_HTTP_RespBody(decmatch T:?)
Pass `exp := tr_HTTP_RespBody(decmatch T:?)` to f_http_transact() in every REST helper that decodes the response body. This ensures both a 2xx status code and a decodable body are required before any decode attempt is made, producing an informative "Unexpected HTTP response" verdict (rather than a confusing DTE) when e.g. an older osmo-s1gw returns HTTP 404 with an empty body for an unsupported endpoint.
eIM_Tests: align RSP version number in HTTP header
The RSP version number in the x-admin-protocol header string should be set to v.2.1.0 (gsma/rsp/v2.1.0), even though the ASN.1 spec we currently use is at version 2.5.
This is to maintain compatibility with older IPAd versions and also stated explicitly in GSMA SGP.32 V2.1, section 6.1.
