test: don't put $BINDIR in $PATH for test-merge-tools.t We call $BINDIR/hg explicitly anyway, so don't need it in $PATH. This fixes failures when running the test --with-hg=/usr/bin/hg, where we pick up /usr/bin/false as merge tool when we expected not to find it.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}