MiniTest 3.3.0 seems broken in some (fairly simple-looking) cases that 3.2.0 didn’t break. The presenting error is below.
It’s claiming that an auto-generated Test Suite doesn’t have a “run” method. I haven’t looked through enough diffs to be sure what the problem is.
A simple workaround is to go back to MiniTest 3.2.0, possibly by adding a “~>3.2.0” to your Gemfile.