lkdtm: Split WARNING into separate tests

There are three paths through the kernel code exception logging:

- BUG (no configurable printk message)
- WARN_ON (no configurable printk message)
- WARN (configurable printk message)

LKDTM was not testing WARN_ON(). This is needed to evaluate the placement
of the "cut here" line, which needs special handling in each of the
three exceptions (and between architectures that implement instruction
exceptions to implement the code exceptions).

Signed-off-by: Kees Cook <keescook@chromium.org>