TYCHO-406 add expression for JVM args in TestMojo

@parameter expression="${tycho.testArgLine}" for
test JVM arguments enables coverage tools to set
instrumentation arguments from separate mojos.

tycho-surefire/maven-osgi-test-plugin/src/main/java/org/codehaus/tycho/osgitest/TestMojo.java

@@ -198,7 +198,7 @@ public class TestMojo extends AbstractMojo {
 	/**
      * Arbitrary JVM options to set on the command line.
      * 
-     * @parameter
+     * @parameter expression="${tycho.testArgLine}"
      */
     private String argLine;