RFR(S): 8226779: [TESTBUG] Test JFR API from Java agent

[mikhailo.seledtsov at oracle.com]

Please review this new test. It is testing interaction between JFR and 
Java Agent,
creating recordings from within Java agent, both premain() and agentmain().
This is also a bit of a stress test since it tests large number of 
recordings from
multiple threads, from the agent, hence I placed it under 
jfr/stress/javaagent.

     JBS: https://bugs.openjdk.java.net/browse/JDK-8226779
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8226779.00/
     Testing:
       1. Running the test itself multiple times on multiple platforms - 
in progress
          (Passed on Mac so far)

Thank you,
Misha