use !cbt_disabled in place of LearnCBT to avoid needless circs

This would make us do testing circuits "even when cbt is disabled by
consensus, or when we're a directory authority, or when we've failed
to write cbt history to our state file lately." (Roger's words.)

This is a fix for 9671 and an improvement in our fix for 5049.
The original misbehavior was in 0.2.2.14-alpha; the incomplete
fix was in 0.2.3.17-beta.