Relocated GreenMailUtilTest.testImapBadEnvelope() to EscapingTest
Renamed testImapBadEnvelope() to testEscapeMessageID()
Relocated GreenMailMimeMessage inner class to it's own class in com.icegreen.greenmail.test.util
Refactored testEscapeMessageID() to use retrieveAndCheck() helper method and GreenMail server instance created by test @Rule