Fix hands-off testing of asebahttp

Automatically start asebadummynode 0 for test-asebahttp. Use a bash script rather than a cmake script since cmake can't execute a process in the background; this works in MacOSX and Ubuntu, and in Windows if compiling under MSYS2 / MINGW.

Explicity use 127.0.0.1 for Aseba::HttpInterface's default dashel target (tcp:;port=33333) to fix testing under Windows.