fix GUI tests on (headless) JDK21 builds #57

Job	Run time
Tests and Analysis (JDK: 8)	14m 26s
Tests and Analysis (JDK: 11)	15m 45s
Tests and Analysis (JDK: 17)	20m 1s
Tests and Analysis (JDK: 21)	17m 55s
Platform Integration (JDK: 8, OS: ubuntu-latest)	1m 55s
Platform Integration (JDK: 8, OS: windows-latest)	3m 51s
Platform Integration (JDK: 8, OS: macOS-latest)	6m 24s
Platform Integration (JDK: 11, OS: ubuntu-latest)	2m 25s
Platform Integration (JDK: 11, OS: windows-latest)	3m 20s
Platform Integration (JDK: 11, OS: macOS-latest)	2m 54s
Platform Integration (JDK: 17, OS: ubuntu-latest)	2m 8s
Platform Integration (JDK: 17, OS: windows-latest)	3m 26s
Platform Integration (JDK: 17, OS: macOS-latest)	4m 55s
Platform Integration (JDK: 21, OS: ubuntu-latest)	3m 9s
Platform Integration (JDK: 21, OS: windows-latest)	4m 10s
Platform Integration (JDK: 21, OS: macOS-latest)	4m 8s
Deploy	3m 52s
Coverage	4m 46s
	1h 59m 30s

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