selftests/lkdtm: Don't clear dmesg when running tests

It is Very Rude to clear dmesg in test scripts. That's because the
script may be part of a larger test run, and clearing dmesg
potentially destroys the output of other tests.

We can avoid using dmesg -c by saving the content of dmesg before the
test, and then using diff to compare that to the dmesg afterward,
producing a log with just the added lines.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>