maintcheck: fix spurious warnings

maintcheck: fix spurious warnings

* t/distcheck-override-infodir.sh: Be sure that valid occurences
of the "aclocal" and "automake" strings, which can confuse the
'sc_tests_plain_automake' check, are protected by leading "#"
characters.
* t/ax/test-lib.sh: Always use '$(...)' for command subtitution,
to avoid triggering the 'sc_tests_command_subst' check; there was
still once place where `...` was used.  While at it, fix a related
comment.
* t/ax/test-defs.in ($sleep): Use creative quoting to avoid
spuriously triggering the 'sc_tests_plain_sleep' check.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>