tests: fix test-patch.t on pickier /bin/sh implementations
FreeBSD sh(1) doesn't accept -d, so we weren't testing what we
expected there. Let's just use a simple Python script instead.
Differential Revision: https://phab.mercurial-scm.org/D3620
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors