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.

tests/test-merge-tools.t

@@ -72,7 +72,7 b' hg merge -r 2'
 override $PATH to ensure hgmerge not visible; use $PYTHON in case we're
 running from a devel copy, not a temp installation
 
-  $ PATH="$BINDIR:/usr/sbin" "$PYTHON" "$BINDIR"/hg merge -r 2
+  $ PATH="/usr/sbin" "$PYTHON" "$BINDIR"/hg merge -r 2
   merging f
   warning: conflicts while merging f! (edit, then use 'hg resolve --mark')
   0 files updated, 0 files merged, 0 files removed, 1 files unresolved
@@ -121,7 +121,7 b" unexecutable file in $PATH shouldn't be "
 
   $ echo "echo fail" > false
   $ hg up -qC 1
-  $ PATH="`pwd`:$BINDIR:/usr/sbin" "$PYTHON" "$BINDIR"/hg merge -r 2
+  $ PATH="`pwd`:/usr/sbin" "$PYTHON" "$BINDIR"/hg merge -r 2
   merging f
   warning: conflicts while merging f! (edit, then use 'hg resolve --mark')
   0 files updated, 0 files merged, 0 files removed, 1 files unresolved
@@ -135,7 +135,7 b" executable directory in $PATH shouldn't "
 
   $ mkdir false
   $ hg up -qC 1
-  $ PATH="`pwd`:$BINDIR:/usr/sbin" "$PYTHON" "$BINDIR"/hg merge -r 2
+  $ PATH="`pwd`:/usr/sbin" "$PYTHON" "$BINDIR"/hg merge -r 2
   merging f
   warning: conflicts while merging f! (edit, then use 'hg resolve --mark')
   0 files updated, 0 files merged, 0 files removed, 1 files unresolved