Support mocking of obfs4 transports in bridge-extrainfo descriptors.

 * FIXES part of #12932 (or at least allows BridgeDB to have regression
   tests for #12932). See https://bugs.torproject.org/12932.
Support mocking of obfs4 transports in bridge-extrainfo descriptors.

 * FIXES part of #12932 (or at least allows BridgeDB to have regression
   tests for #12932). See https://bugs.torproject.org/12932.
Merge remote-tracking branch 'tpo-isis/fix/12932' into fix/12932

Conflicts:
	leekspin/extrainfo.py
Merge branch 'fix/12932' into develop