I still occasionally see these test failures on Windows, even after the fix for JDK-8246099, which bumped the timeout value for the test to 25 seconds:

test.sandbox.SandboxAppTest > testFXNonApp FAILED
    java.lang.Exception: test timed out after 25000 milliseconds
        at java.base@15/java.lang.ProcessImpl.waitForInterruptibly(Native Method)
        at java.base@15/java.lang.ProcessImpl.waitFor(ProcessImpl.java:550)
        at app//test.sandbox.SandboxAppTest.runSandboxedApp(SandboxAppTest.java:70)
        at app//test.sandbox.SandboxAppTest.runSandboxedApp(SandboxAppTest.java:52)
        at app//test.sandbox.SandboxAppTest.testFXNonApp(SandboxAppTest.java:110)

test.sandbox.SandboxAppTest > testJFXPanelApp SKIPPED

Gradle Test Executor 124 finished executing tests.

> Task :systemTests:test

test.sandbox.SandboxAppTest > testFXApp FAILED
    java.lang.Exception: test timed out after 25000 milliseconds
        at java.base@15/java.lang.ProcessImpl.waitForInterruptibly(Native Method)
        at java.base@15/java.lang.ProcessImpl.waitFor(ProcessImpl.java:550)
        at app//test.sandbox.SandboxAppTest.runSandboxedApp(SandboxAppTest.java:70)
        at app//test.sandbox.SandboxAppTest.runSandboxedApp(SandboxAppTest.java:52)
        at app//test.sandbox.SandboxAppTest.testFXApp(SandboxAppTest.java:105)