Without syncing the two threads with a lock, the spec could accidentally
pass even if the code is broken. It was accidentally removed during
refactoring in b0fa1464f.
In order to make the spec fail if the controller was not thread safe, it
uses breakpoints. One of those breakpoints was set for a method that has
now been removed.
I changed the method that is used for the breakpoint and changed `allow`
to `expect` so that this spec will fail if we remove that method as
well. Future version of Rspec will check if a mocked method actually
exists but our version just mocks it anyway. This is one way how specs
can become invalid after refactoring.
