selftests/landlock: Redefine TEST_F() as TEST_F_FORK()

This has the effect of creating a new test process for either TEST_F()
or TEST_F_FORK(), which doesn't change tests but will ease potential
backports.  See next commit for the TEST_F_FORK() merge into TEST_F().

Cc: Günther Noack <gnoack@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Will Drewry <wad@chromium.org>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>