t/scripts/strict.t: Test our checks + missing "use warnings"

Enable the test for missing "use warnings;" in all files.  Also add
"strict_ok" and "warnings_ok" tests for our checks.  The "syntax_ok"
does not like our checks, but they are covered by "check-load.t"
anyway.

Signed-off-by: Niels Thykier <niels@thykier.net>