maintcheck: guard against `...` for command substitution in test cases

maintcheck: guard against `...` for command substitution in test cases

But still allow them in configure.ac, Makefile.am and shell scripts
created or used inside test cases itself, because Autoconf (as of
version 2.69) does not yet ensure that $CONFIG_SHELL will be set to
a proper POSIX shell.

* syntax-checks.mk (sc_tests_command_subst): New check.
(syntax-check_rules): Add it.
* t/tap-global-log.sh: Minimal tweakings to avoid triggering the
new maintainer check.

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