Mention the use of @WebAppConfiguration with Spock and make the JUnit examples consistent

[wilkinsona]

The section on Spock makes no mention of @WebAppConfiguration. The section on JUnit uses @WebAppConfiguration inconsistently in its various examples. We should probably also mention that @IntegrationTest can be used with Spock.

[wilkinsona]

I don't think the JUnit examples need to be made consistent. The examples are different as they're describing different options depending on what's being tested and how the testing is being performed.