Issue588: set the property testfx.robot to glass when running the TestFX based integration tests

Author: afester

richtextfx/build.gradle

@@ -20,6 +20,7 @@ testSets {
 
 check.dependsOn integrationTest
 integrationTest.mustRunAfter test
+integrationTest.systemProperty "testfx.robot", "glass"
 if (gradle.gradleVersion.substring(0, 1) >= "4") {
     // required for Gradle 4 to see custom integrationTest test suite
     integrationTest.testClassesDirs = sourceSets.integrationTest.output.classesDirs