selftests/livepatch: Use "comm" instead of "diff" for dmesg

BusyBox diff doesn't support the GNU diff '--LTYPE-line-format' options
that were used in the selftests to filter older kernel log messages from
dmesg output.

Use "comm" which is more available in smaller boot environments.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Reviewed-by: Yannick Cote <ycote@redhat.com>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20200710183745.19730-1-joe.lawrence@redhat.com