8222888: [TESTBUG] docker/TestJFREvents.java fails due to "RuntimeException: JAVA_MAIN_CLASS_* is not defined"

[Erik Gahlin]

Looks good

Erik

> Please review this change that makes a test more robust. The test 
> originally relied on the fact that JAVA_MAIN_CLASS variable
> is always present when running jtreg tests. This assumption is wrong, 
> hence I reworked the test to define its own unique
> environment variable.
>
> In order to introduce environment variable I reworked the 
> DockerTestUtils a little bit, by splitting the
> dockerRunJava() method into 2: buildJavaCommand() to build the 
> command; dockerRunJava() uses buildJavaCommand()
> then runs the command. This allowed me to introduce the test 
> environment variable to the child process.
> The behavior of dockerRunJava() should not be affected by this change, 
> it is a simle split.
>
>     JBS: https://bugs.openjdk.java.net/browse/JDK-8222888
>     Webrev: http://cr.openjdk.java.net/~mseledtsov/8222888.00/
>     Testing:
>         1. Ran hotspot docker tests on Linux-x64 machine with docker 
> enigne configured.
>            Ran both via jtreg directly and via make
>            All PASS
>
> Thank you,
> Misha
>