Restore exact behavior of let/subject/before(:all) that 2.12 (and before) had.

Created by: myronmarston

This isn't pretty, but it's just for 2.13.x patch releases, to fix the regression as per SemVer. Read the inline comments for more details.

Feedback welcome, but I'll probably merge and release 2.13.1 with this in the next few days.