Force all tests to fail with a consistent message.

Per the README: after running ./gradlew removeSolutions, "all the *Spock
tests in the com.example.interaction package should fail" however,
_2InvocationCardinalitySpock."should not match any" does not fail with
the solutions removed.

Making all tests fail by replacing the solutions with an explicit
fail(), in addtion to moving the assertion for this one test into the
"//SNIPPET" block will make sure that all tests do fail and remove any
confusion.