JDK 19/20 CI intermittent failures / flaky test

https://github.com/java-native-access/jna/actions/runs/5801959465/job/15727351939
https://github.com/java-native-access/jna/actions/runs/5875801992/job/15948144145?pr=1544

    [junit] Testsuite: com.sun.jna.CallbacksTest
    [junit] JNA: Can't attach native thread to VM for callback: -1 (check stacksize for callbacks)
    [junit] Tests run: 53, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 4.508 sec
    [junit] 
    [junit] Testcase: testInvokeCallback(com.sun.jna.CallbacksTest):	FAILED
    [junit] Existing native function wrapper should be reused expected same:<Proxy interface to native function@0x7f4d400093b0 (com.sun.jna.CallbacksTest$TestLibrary$Int32CallbackX)> was not:<Proxy interface to native function@0x7f4d400093b0 (com.sun.jna.CallbacksTest$TestLibrary$Int32CallbackX)>
    [junit] junit.framework.AssertionFailedError: Existing native function wrapper should be reused expected same:<Proxy interface to native function@0x7f4d400093b0 (com.sun.jna.CallbacksTest$TestLibrary$Int32CallbackX)> was not:<Proxy interface to native function@0x7f4d400093b0 (com.sun.jna.CallbacksTest$TestLibrary$Int32CallbackX)>
    [junit] 	at com.sun.jna.CallbacksTest.testInvokeCallback(CallbacksTest.java:969)
    [junit] Test com.sun.jna.CallbacksTest FAILED
    [junit] 	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)

Start with com.sun.jna.CallbacksTest.testInvokeCallback at CallbacksTest.java:969 and review the linked CI runs for the JDK 19/20 failures. Reproduce the intermittent callback-thread and native function wrapper failure, then verify the test passes reliably on the affected JDK versions.