[REEF-79]: Add a script that enables running REEF-Tests on YARN.

Changes:
* Added the script (thanks to @dafrista, @markusweimer, @bgchun)
* Added test-jar to include test code in test-jar-with-dependencies, which we will run tests against



How to run reef-tests on YARN:

1. Build reef by running mvn clean install -DskipTests 
2. Build reef-tests jar by running mvn jar:test-jar from the reef-tests directory
3. Run the tests by running bin/runyarntests.sh

This is actually necessary. Although the test-jar is not directly used in the script, it seems that without it the generated `test-jar-with-dependencies` will not contain the classes in the reef-tests/test directory. 

Hence, when we run the script without `test-jar`, we get a complain that the class `AllTestsSuite` cannot be found. You can reproduce this behavior by running the script without generating `test-jar`. Also you can do `vi $REEF_HOME/reef-tests/target/reef-tests-0.10-incubating-SNAPSHOT-test-jar-with-dependencies.jar` and see that the test code is not included in the jar.

Also please refer to http://stackoverflow.com/a/11787964, from which I got the idea. The answer says "This assembly relies on the test-jar being created as part of the module build".