OBMFULL-5777 modernize ErrorMailerTest
OBMFULL-5777 modernize EventChangeMailerTest
OBMFULL-5777 never use javax.mail.Session.getDefaultSession

	This method despite it takes arguments construct a Singleton shared
	by the whole JVM and doesn't take arguments into account after
	the first call.

	We need to create a new instance of Session everytime we need one.