JavaTest/JTREG tests hanging at exit on Windows

This failure was spotted in the 2014-03-18 RT_Baseline nightly:

com/sun/jdi/SDENullTest.java

    This test failed due to a timeout during the test execution phase.
    The test compilation phase went quickly:

----------messages:(3/242)*----------
command: compile -g C:\\local\\aurora\\sandbox\\sca\\vmsqe\\testbase\\jtreg\\nightly\\rt_baseline\\JT_JDK\\test\\com\\sun\\jdi\\SDENullTest.java
reason: User specified action: run compile -g SDENullTest.java
elapsed time (seconds): 1.894

    so that's an indication that the test machine is not overloaded.

    Here's the "messages", "System.out" and "System.err" sections
    of the .jtr file:

----------messages:(3/113)----------
command: main SDENullTest
reason: User specified action: run main SDENullTest
elapsed time (seconds): 960.04
----------System.out:(7/559)*----------
-- Added debuggeeVM options from file C:\\local\\aurora\\sandbox\\results\\workDir\\classes\\com\\sun\\jdi\\@debuggeeVMOptions: -Xmixed -client -XX:MaxRAMFraction=8 -XX:+CreateMinidumpOnCrash -XX:NativeMemoryTracking=detail -XX:ReservedCodeCacheSize=256M
JVM version:1.8.0-fastdebug
JDI version: 1.6
JVM description: Java Debug Interface (Reference Implementation) version 1.6
Java Debug Wire Protocol (Reference Implementation) version 1.6
JVM Debug Interface version 1.2
JVM version 1.8.0-fastdebug (Java HotSpot(TM) Client VM, mixed mode, sharing)
----------System.err:(3/62)----------
run args: [SDENullTarg]
SDENullTest: passed
STATUS:Passed.

    The "messages" section shows the 16 minute timeout. The "System.out"
    section shows the usual version info for a JDI test. The "System.err"
    section shows "SDENullTest: passed" and "STATUS:Passed". The test
    has the following code at the end of the test:

    76 if (!testFailed) {
    77 println("SDENullTest: passed");
    78 } else {
    79 throw new Exception("SDENullTest: failed");
    80 }

    so this test thinks it passed and the "STATUS:Passed" line indicates
    that the JavaTest/JTREG harness agrees. So what happened?