signal/testing: Don't look for __SI_FAULT in userspace

Fix the debug print statements in these tests where they reference
si_codes and in particular __SI_FAULT.  __SI_FAULT is a kernel
internal value and should never be seen by userspace.

While I am in there also fix si_code_str.  si_codes are an enumeration
there are not a bitmap so == and not & is the apropriate operation to
test for an si_code.

Cc: Dave Hansen <dave.hansen@linux.intel.com>
Fixes: 5f23f6d0 ("x86/pkeys: Add self-tests")
Fixes: e754aedc ("x86/mpx, selftests: Add MPX self test")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>