skype: add ATTACH_GDB env flag, extend hacking instructions with info on pexpect/gdb

As I did a rewrite of skyped, started getting reproducible segfaults in plugin code during tests, so thought that maybe something like proposed might be useful in general?

Patches allow running `ATTACH_GDB=true ./test.py` and get the backtraces in "t/gdb-<pid>.log", as (now) documented in HACKING.
Also noted/fixed the absense of "pexpect" requirement there and added a few lines on how to run specific test, which might useful as these tests tend to run fairly slow (not just testing 3-liner methods).