Declare empty tycho.testArgLine property

This used to work by accident thanks to the fact that
jacoco-maven-plugin changed the testing parameters thus this unresolved
variable went unnoticed.

pom.xml

@@ -12,6 +12,7 @@
 		<jgit.dirtyWorkingTree>error</jgit.dirtyWorkingTree>
 		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 		<ui.test.vmargs />
+		<tycho.testArgLine />
 	</properties>
 
 	<modules>