RFR 8217347 [TESTBUG] runtime/appcds/jvmti/InstrumentationTest.java timed out

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8217347
http://cr.openjdk.java.net/~iklam/jdk13/8217347-appcds-InstrumentationTest-timeout.v01/

This test case used VirtualMachine.list() to find the pid of the the child
process. However, as David Holmes commented in the bug report, it's
suspected that this call may lead to timeout.

In any case, since Process.pid() has been added since JDK 9, it's better 
to call this
API directly. That way, we can avoid unnecessary dependency on 
VirtualMachine.list().

Tested with tiers{1,2,3}. Also removed some unnecessary @module and 
import lines.

Thanks
- Ioi