maintcheck: placate maintainer-check and grep 3.8.

maintcheck: placate maintainer-check and grep 3.8.

* maintainer/syntax-checks.mk (sc_rm_minus_f): no \ before -
(diagnosed by grep 3.8).
* t/comment12.sh: use $(...) instead of `...`.
* t/comments-escaped-in-var.sh: use AUTOMAKE_fails and grep
for expected warning message.
* t/list-of-tests.mk (handwritten_TESTS): add t/py-compile-files.sh.
* t/subdir-add2-pr46.sh (.NOTPARALLEL): add; showed up with
parallelized internal make. Seems unreproducible.