RFR(S): 8213917: [TESTBUG] Shutdown JFR event is not covered by test

[mikhailo.seledtsov at oracle.com]

Please review:
     JBS: https://bugs.openjdk.java.net/browse/JDK-8213917
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8213917.00/
     Testing:
       1. Ran the affected test locally multiple times: PASS
       2. Ran the affected test and entire open/test/jdk/jdk/jfr/event/ 
on an automated multi-platform system.
          Ran several times/repeats to make sure test is stable.
          All PASS

Thank you,
Misha