RFR(T): Trivial: 8217748: [TESTBUG] Exclude TestSig test case from JFR TestShutdownEvent

[mikhailo.seledtsov at oracle.com]

     JBS: https://bugs.openjdk.java.net/browse/JDK-8217748
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8217748.00/index.html
     Testing: Ran the test locally (Mac), PASSED
              Checked the logs to make sure TestSig cases are not executed

Thank you,
Misha