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
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif