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>