Set tycho.useJDK=SYSTEM for org.eclipse.pde.junit.runtime

We already use that for the API tools and as java 1.8 is vanish from
github we want to disable it here as well.

Fix #2099

Author: laeubi

ui/org.eclipse.pde.junit.runtime/build.properties

@@ -19,3 +19,4 @@ bin.includes = .,\
                META-INF/
 src.includes = about.html
 output.. = bin/
+pom.model.property.tycho.useJDK=SYSTEM