selftests/lkdtm: Add configs for stackleak and "after free" tests

Add config options which are needed for LKDTM sub-tests:
STACKLEAK_ERASING test needs GCC_PLUGIN_STACKLEAK config.
READ_AFTER_FREE and READ_BUDDY_AFTER_FREE tests need
INIT_ON_FREE_DEFAULT_ON config.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220517132932.1484719-1-usama.anjum@collabora.com