Illegal reflective access warning on JDK11

Hi, on java 11 I get the following test warnings due to illegal reflective access:

```
WARNING: Illegal reflective access by org.assertj.swing.dependency.fest_reflect.util.Accessibles$SetAccessibleAction (file:/home/hydrologis/.m2/repository/org/assertj/assertj-swing/3.8.0/assertj-swing-3.8.0.jar) to method javax.swing.SwingUtilities.appContextGet(java.lang.Object)
at org.assertj.swing.dependency.fest_reflect.util.Accessibles$SetAccessibleAction.run(Accessibles.java:70)
at org.assertj.swing.dependency.fest_reflect.util.Accessibles$SetAccessibleAction.run(Accessibles.java:60)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at org.assertj.swing.dependency.fest_reflect.util.Accessibles.setAccessible(Accessibles.java:57)
at org.assertj.swing.dependency.fest_reflect.util.Accessibles.makeAccessible(Accessibles.java:46)
at org.assertj.swing.dependency.fest_reflect.method.Invoker.invoke(Invoker.java:111)
at org.assertj.swing.edt.FailOnThreadViolationRepaintManager.currentRepaintManager(FailOnThreadViolationRepaintManager.java:82)
at org.assertj.swing.edt.FailOnThreadViolationRepaintManager.installNew(FailOnThreadViolationRepaintManager.java:94)
at org.assertj.swing.edt.FailOnThreadViolationRepaintManager.install(FailOnThreadViolationRepaintManager.java:57)
at org.geotools.swing.testutils.GraphicsTestBase.baseSetUpOnce(GraphicsTestBase.java:54)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:367)
at org.apache.maven.surefire.junit4.JUnit4Provider.executeWithRerun(JUnit4Provider.java:274)
at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:238)
at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:161)
at org.apache.maven.surefire.booter.ForkedBooter.invokeProviderInSameClassLoader(ForkedBooter.java:290)
at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:242)
at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:121)
```

Inizia da org.assertj.swing.dependency.fest_reflect.util.Accessibles.java e org.assertj.swing.edt.FailOnThreadViolationRepaintManager.java, seguendo la chiamata da GraphicsTestBase.java:54 con JDK 11. Esegui i test interessati e traccia l’avviso di accesso riflessivo; il lavoro è completato quando i test terminano senza questo avviso di accesso illegale.

Scritto dal modello di indicizzazione a partire dal testo della issue.