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

[mikhailo.seledtsov at oracle.com]

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