We get a fatal exception when running any system test (whether or not they are robot tests) on the macOS / metal graphics pipeline, after which the test hangs indefinitely.

To reproduce:

$ gradle --info -PFULL_TEST=true :systemTests:test
...
DefaultExceptionHandlerTest STANDARD_ERROR
    RenderJob.run: internal exception
    java.lang.UnsatisfiedLinkError: Invalid URL for class: file:/Users/kcr/dev/javafx/jfx-metal/jfx/rt/build/shims/javafx.graphics/com/sun/prism/mtl/MTLContext.class
     at javafx.graphics@23-internal/com.sun.prism.mtl.MTLContext.<clinit>(MTLContext.java:127)
     at javafx.graphics@23-internal/com.sun.prism.mtl.MTLResourceFactory.<init>(MTLResourceFactory.java:52)
     at javafx.graphics@23-internal/com.sun.prism.mtl.MTLPipeline.getResourceFactory(MTLPipeline.java:90)
     at javafx.graphics@23-internal/com.sun.prism.mtl.MTLPipeline.getDefaultResourceFactory(MTLPipeline.java:82)
     at javafx.graphics@23-internal/com.sun.prism.GraphicsPipeline.getDefaultResourceFactory(GraphicsPipeline.java:155)
     at javafx.graphics@23-internal/com.sun.javafx.tk.quantum.QuantumRenderer.lambda$createResourceFactory$2(QuantumRenderer.java:163)
     at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:572)
     at java.base/java.util.concurrent.FutureTask.runAndReset(FutureTask.java:358)
     at javafx.graphics@23-internal/com.sun.javafx.tk.RenderJob.run(RenderJob.java:58)
     at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
     at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
     at javafx.graphics@23-internal/com.sun.javafx.tk.quantum.QuantumRenderer$PipelineRunnable.run(QuantumRenderer.java:126)
     at java.base/java.lang.Thread.run(Thread.java:1583)