selftests/lkdtm: Reset WARN_ONCE to avoid false negatives

Since we expect to see warnings every time for many tests, just reset
the WARN_ONCE flags each time the script runs.

Fixes: 46d1a0f0 ("selftests/lkdtm: Add tests for LKDTM targets")
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200625203704.317097-4-keescook@chromium.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>