Fix double test reporting.

Clear the test-status.lisp-expr before running impure tests, otherwise
a premature exit will duplicate the previous test results.
grab-bag of walker fixes

- define a specific load-time-value walker function.  It's wrong, but
  at least we now know it's wrong;
- all special-operators are fboundp by design, so don't exclude
  fboundp special operators from our walker consistency check
- untangle what the CONTEXT argument to walker function means
  (and adapt WALK-TAGBODY-1 accordingly)
- delete unused function WALK-BINDINGS-2
- test (SETQ (X) 3) not expanding (X) macro
- no such thing as NESTED-WALK-FORM any more