Make the testing pluggable

- Add Spock tester along with another test case
- Show that "spring test" can support more than one test suite at the same time
- Add autoconfiguration to CLI so that Spock is add automatically.

TODO: This functionality still doesn't work after installing "spring".
CLI has junit and spock removed so those jars don't end up in spring-boot-cli,
but I can't seem to invoke the tester without it complaining that the @Grab'd
test stuff is unloadable.