CxxUtils: Update stacktrace test for newer lcg versions.

With newer lcg versions, the system addr2line doesn't work because it's too old. Normally that doesn't matter, because we usually use eu-addr2line, which does work. However, the stacktrace unit test forces the use of /usr/bin/addr2line (as the output formats are slightly different).

Change stacktrace so that if we don't find /usr/bin/eu-addr2line we search PATH for addr2line, rather than hardcoding it. Also adjust the unit test so that it should work with either addr2line or eu-addr2line.