selftests/lkdtm: Add way to repeat a test

Some LKDTM tests need to be run more than once (usually to setup and
then later trigger). Until now, the only case was the SOFT_LOCKUP test,
which wasn't useful to run in the bulk selftests. The coming stack canary
checking needs to run twice, so support this with a new test output prefix
"repeat".

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20211022223826.330653-2-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>