[15] RFR(T) 8242370: Enable JFR TestGCPhaseConcurrent test for Shenandoah

[Zhengyu Gu]

JDK-8241984 fixed top level concurrent event in Shenandoah, let's enable 
this test to ensure no regression in the future.

Bug: https://bugs.openjdk.java.net/browse/JDK-8242370
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8242370/webrev.00/

Test:
   TestGCPhaseConcurrent.java test on Linux x86_64


Thanks,

-Zhengyu