tests: sniff for /usr/local/bin/gmake and use it in test-fuzz-targets.t
This isn't as robust as it probably should be, but for now it'll get
the job done on the buildbots.
Differential Revision: https://phab.mercurial-scm.org/D5269
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\nOK.