I am interested in running all tests locally, including GUI ones.
When starting “Run Test…” from IntelliJ, the only test-package that does not pass, is
jabref/src/test/java/org/jabref/gui. Most of its tests throw
java.lang.IllegalStateException: Application launch must not be called more than once.
Running these tests, jabref starts once, but nothing happens to the GUI, hence I close it and further ui-tests fail as described above. I am unsure what to do here. 
I then consulted
README.md, advising me to run
gradlew integrationTest, instead. Unfortunately, this results in
FAILURE: Build failed with an exception.
* What went wrong:
Task 'integrationTest' not found in root project 'JabRef'
Indeed, I cannot find an
integrationTest task in build.gradle.
Therefore, I wonder what the state of running UI-tests is. In what forms may I contribute to automatically testing them?
: This is the other error I get:
Prism-ES2 Error : GL_VERSION (major.minor) = 1.4 10:02:49.886 [AWT-EventQueue-0] WARN org.jabref.JabRefGUI - There seem to be problems with OpenJDK and the default GTK Look&Feel. Using Metal L&F instead. Change to another L&F with caution. File: grouptree.fxml not found, attempting with camel case Sep 16, 2017 10:02:54 AM javafx.fxml.FXMLLoader$ValueElement processValue WARNING: Loading FXML document with JavaFX API of version 8.0.112 by JavaFX runtime of version 8.0.60 File: grouptree.css not found, attempting with camel case