Make a test case less sensitive to the contents of $TMP.

spec/command-sub.test.sh

@@ -29,7 +29,7 @@ echo `echo -n l; echo -n s`
# Inner `` are escaped!  # Not sure how to do triple..  Seems like an unlikely
# use case.  Not sure if I even want to support this!
echo X > $TMP/000000-first
echo `\`echo -n l; echo -n s\` $TMP | head -n 1`
echo `\`echo -n l; echo -n s\` $TMP | grep 000000-first`
# stdout: 000000-first

### Making command out of command sub should work