Teach predtest.c about BooleanTest

There are lots of things we can prove based on/for IS [NOT]
{TRUE,FALSE,UNKNOWN} clauses -- particularly how they related to boolean
variables, NOT clauses, and IS [NOT] NULL clauses.

There are a few other more generic improvements as well, such as the
ability for clause_is_strict_for to recurse into NOT clauses and
adjusting proofs refutation based on implication to know that if we can
prove strong refutation we can also prove weak refutation by default.

Author: jcoleman