[REEF-1865] Allow runyarntests.sh run individual unit tests

JIRA:
  [REEF-1865](https://issues.apache.org/jira/browse/REEF-1865)

Pull Request:
  Closes #1366
diff --git a/bin/runyarntests.sh b/bin/runyarntests.sh
index 54674e7..64b6095 100755
--- a/bin/runyarntests.sh
+++ b/bin/runyarntests.sh
@@ -22,6 +22,6 @@
 DEPENDENCY_JAR=`echo $REEF_HOME/lang/java/reef-tests/target/reef-tests-*-test-jar-with-dependencies.jar`
 CLASSPATH=`yarn classpath`
 
-CMD="java -cp $YARN_CONF_DIR:$DEPENDENCY_JAR:$CLASSPATH org.junit.runner.JUnitCore org.apache.reef.tests.AllTestsSuite $*" 
+CMD="java -cp $YARN_CONF_DIR:$DEPENDENCY_JAR:$CLASSPATH org.junit.runner.JUnitCore ${*:-org.apache.reef.tests.AllTestsSuite}"
 echo $CMD
 $CMD