Add PATH_BIAS_TESTING purpose to specs

Similar to legacy/trac#33639 (moved) I found

2020-03-17 21:26:35,977 stem [INFO] CIRC event had an unrecognized purpose (PATH_BIAS_TESTING). Maybe a new addition to the control protocol? Full Event: 'CIRC 322 CLOSED $8E915C44FD55A433CCAFACF4902BF1B2B16DD36C~snap277,$ECE90C4159D916921F7C8DDF4D088AC3808C521D~piss PURPOSE=PATH_BIAS_TESTING TIME_CREATED=2020-03-17T20:26:15.358481 REASON=FINISHED'

in my stem logs and it seems PATH_BIAS_TESTING needs to get added to the specs as a "new" PURPOSE so stem can pick this up, too.

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