maint: make some shell and perl scripts executable in 'tests/'

maint: make some shell and perl scripts executable in 'tests/'

Some newer test scripts - partially ones from me - are not executable.
It does not seem to be a problem, but for consistency and to avoid
future problems on unusual platforms or shells change the permissions
by adding the executable bit.

* cfg.mk (sc_tests_executable): Add new syntax-check rule to ensure
that all test scripts are executable.
* tests/df/df-output.sh: Change file mode from 644 to 755.
* tests/du/threshold.sh: Likewise.
* tests/factor/run.sh: Likewise.
* tests/init.sh: Likewise.
* tests/misc/csplit-suppress-matched.pl: Likewise.
* tests/misc/numfmt.pl: Likewise.
* tests/tail-2/retry.sh: Likewise.