JbpmPreprocessingUnitCommonTest.testReadFile() uses System.lineSeparator

In 6.4.x branch of jbpm-designer repo there is test JbpmPreprocessingUnitCommonTest.testReadFile() that uses System.lineSeparator() method, which is available only from jdk 1.7. So when running this test on jdk 1.6, the test doesn't compile. I will make PR for this.