Strip identity attributes from offers before the engine

The NetherNet spec instructs removing a=identity before SDP reaches
WebRTC: the assertion is signaling layer metadata, and current client
offers always carry one. The engine tolerated it in live testing, but
stripping removes the dependence on that tolerance. The unstripped
offer stays available above the backend for future validation.

Also fixes the test only constructor ambiguity introduced with the
TLS supplier overload, which broke compileTestJava without failing
JitPack builds.