wvtest: WVFAIL(cond) returns cond

wvtest: WVFAIL(cond) returns cond

Previously WVFAIL returned !cond. In my optinion it was confusing
because the statements
  if (cond) print_error();
and
  if (WVFAIL(cond)) print_error();
behaved differently. Now, they are functionally equivalent. In the
implementation we use C extensions typeof() and "statement expression"
to avoid duplication of side-effects caused by cond evaluation.