lkdtm: Enable DOUBLE_FAULT on all architectures

lkdtm: Enable DOUBLE_FAULT on all architectures

Where feasible, I prefer to have all tests visible on all architectures,
but to have them wired to XFAIL. DOUBLE_FAIL was set up to XFAIL, but
wasn't actually being added to the test list.

Fixes: cea23efb4de2 ("lkdtm/bugs: Make double-fault test always available")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210623203936.3151093-7-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>