RFR(S): 8213914: [TESTBUG] Several JFR VM events are not covered by tests

[Mikhailo Seledtsov]

Please review this fairly small change that enables/adds testing for 
several JVM JFR events that were not covered.
     JBS: https://bugs.openjdk.java.net/browse/JDK-8213914
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8213914.00/index.html
     Testing:
         Ran open/test/jdk/jdk/jfr/ on multiple platforms
         All PASS


Thank you,
Misha