[TESTBUG] docker/TestJFREvents.java fails due to "RuntimeException: JAVA_MAIN_CLASS_* is not defined"

The following test failed in the JDK13 CI:

containers/docker/TestJFREvents.java

Here's a snippet of the log file:

----------System.err:(14/856)----------
java.lang.RuntimeException: JAVA_MAIN_CLASS_* is not defined
at TestJFREvents.getTestEnvironmentVariable(TestJFREvents.java:130)
at TestJFREvents.testEnvironmentVariables(TestJFREvents.java:142)
at TestJFREvents.main(TestJFREvents.java:73)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:567)
at com.sun.javatest.regtest.agent.MainActionHelper$AgentVMRunnable.run(MainActionHelper.java:298)
at java.base/java.lang.Thread.run(Thread.java:835)

JavaTest Message: Test threw exception: java.lang.RuntimeException
JavaTest Message: shutting down test

result: Failed. Execution failed: `main' threw exception: java.lang.RuntimeException: JAVA_MAIN_CLASS_* is not defined


test result: Failed. Execution failed: `main' threw exception: java.lang.RuntimeException: JAVA_MAIN_CLASS_* is not defined