Tidy up a number of sandboxed signal-catching test cases:

- Use separate test cases for load vs. store permission-failure exceptions.
- Fix up a number of expected signal numbers, MIPS exception codes, and CP2
  exception codes.
- Ensure that the correct signal is re-enabled in several 'catch' cases.

All test cases pass except for those involving jumping to a NULL pointer
within a sandbox, which appears to fire a TRAP exception instead of a TLB
miss exception; this case is correct outside of sandboxes and it is as-yet
unclear if this is a hardware or software bug.