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.
Fixing spelling errors.

Adding a note to the solution for _2InvocationCardinalitySpock."should
match any times including zero" to give more information about the "_*"
construct and how/where it should be used.
Adding Mockito test implementations for
_3InvocationResponseMockito.shouldOptionallyThrowExceptionBasedOnInputArguments
and shouldUseArgumentsToCalculateResponse.
Replacing setUp methods with @InjectMocks.
Cleaning up unused imports and some formatting.
Incorporating pull request feedback:

Removing 'should match any times including zero'
Moving the values == [] assert back outside the //SNIPPET
Merge pull request #1 from Deravel/master

Feedback after running through the onboarding exercises