Module for tests

Moves all test classes to own module:
- Package names for test classes are still the same to stay compatible with existing properties files
- Renamed old tests package to test (so we have a new package name for the test related stabilizer framework classes)
- Moved test exceptions to new sub-package
- Little cleanup of stabilizer pom.xml
- Wrote now failing test class for AgentSmokeTest since test classes are not available in stabilizer module anymore (couldn't test this though)

Due to some problems with static imports after the movement I fixed some minor issues:
- Removed sleepMillis() method from TestUtils (default Utils class has this method)
- Moved some commonly used helper classes from helper sub-package to base helper package
- Changed name for specific zipfian utils class from Utils to ZipfianUtils and made it package private
- Some assertXXX() got reformatted during fixing static imports

There are still a lot of Hazelcast dependencies in the stabilizer itself, but those will not be solved in this PR (it's big enough).