ktest: force $buildonly = 1 for 'make_warnings_file' test type

ktest: force $buildonly = 1 for 'make_warnings_file' test type

The test type "make_warnings_file" should have no mandatory configuration
parameters other than the ones required by the "build" test type, because
its purpose is to create a file with build warnings that may or may not be
used by other subsequent tests. Currently, the only way to use it as a
stand-alone test is by setting POWER_CYCLE, CONSOLE, SSH_USER,
BUILD_TARGET, TARGET_IMAGE, REBOOT_TYPE and GRUB_MENU.

Link: https://lkml.kernel.org/r/20240315-ktest-v2-1-c5c20a75f6a3@marliere.net
Cc: John Hawley <warthog9@eaglescrag.net>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>