RFR(S): 8241033: Create test library utility for getting JFR streaming repository

[mikhailo.seledtsov at oracle.com]

Please review this change that introduces a test library utility for 
obtaining a JFR streaming repository.
Several tests use this functionality, and each implement their own 
methods. This change unifies the functionality, and also
improves it including added diagnostics.

     JBS: https://bugs.openjdk.java.net/browse/JDK-8241033
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8241033.02/
     Testing:
        Ran open/test/jdk/jdk/jfr/api/consumer/streaming/
            PASS


Thank you,
Misha