RFR(S): 8167493: Test that JFR event can be retransformed by an agent

[mikhailo.seledtsov at oracle.com]

Please review this change that adds a new test to verify that a 
user-defined JFR event can be tramsformed by an agent.
This test uses a class transformer to add a callback into the <init> of 
the event class, generates/records the event,
and verifies that the added instrumentation callback was actually called.

Also, the JavaAgentBuilder testlib utility was extended to accept 
additional manifest attributes.

     JBS: https://bugs.openjdk.java.net/browse/JDK-8167493
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8167493.02/
     Testing:
         1. Ran the newly created test: PASS
         2. Ran all tests using JavaAgentBuilder: PASS


Thank you,
Misha