lkdtm: Update tests for memcpy() run-time warnings

Clarify the LKDTM FORTIFY tests, and add tests for the mem*() family of
functions, now that run-time checking is distinct.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>