Cleanup and modernize mockrunner-servlet unit tests

[steinarb]

@cemartins Could you take a look at this, please?

The changes are:

1. A change of maven dependency scope from provided to test for junit 4 in mockrunner-servlet (I got a lot of incomprehensible build errors when I pulled in junit jupiter as a test scope dependency)
2. Move example tests from src/main/java to src/test/java to be treated right by surefire
3. Remove JUnit 4 annotations from BasicServletTestCaseAdapter (which was what required JUnit 4 to be in the provided scope), the annotations were not required by the examples in 2 above, sin the examples call methods in the base class anyway
4. Add JUnit jupiter and assertj as maven dependencies in mockrunner-servlet/pom.xml
5. Change all unit tests in mocrunner-servlet from JUnit 3 test built with Junit 4, to JUnit jupiter tests, using assertj to do asserts on collections
6. Change the example tests in 2 from JUnit 4 to JUnit jupiter

(this is sort of a preparation to fixing issue 69. I needed unit tests for that, and the existing unit tests of mockrunner-servlet were... weird...)

[cemartins]

Glad that you're taking this on!
Changes look good to me.

[steinarb]

Hi this was the wrong PR. I didn't pay attention and made a PR against my mockrunner repo which is sort of useless, so I closed this one and made a new one in the right repo. Ie. mockrunner#93 that I see you've also looked at.

Sorry about that!