tests: prefer using 'is_newest' over 'ls -t' hacks

tests: prefer using 'is_newest' over 'ls -t' hacks

To verify that a file 'new' is newer than the file 'old', it's much
cleaner to use "is_newest new old" rather than the more clumsy
"ls -t". Adjust several of our tests accordingly.

* syntax-checks.mk (sc_tests_ls_t): New check, guard against uses
of "ls -t" and similar.
(syntax_check_rules): Add it.
* t/maken.sh: Adjust.
* t/autohdr4.sh: Likewise.
* t/extradep2.sh: Likewise.
* t/lex-depend-cxx.sh: Likewise.
* t/extradep.sh: Likewise.
* t/yacc-depend2.sh: Likewise.
* t/yacc-pr204.sh: Likewise.
* t/lex-pr204.sh: Likewise.
* t/yacc8.sh: Likewise.
* t/acloca13.sh: Likewise.
* t/lex-depend.sh: Likewise, and enhance.
* t/acloca14.sh: Likewise.
* t/aclocal7.sh: Likewise.

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