RFR(XS): 8157468: gc/testlibrary contains a lot of duplicated code

[Kirill Zhaldybin]

Dear all,

Could you please review this fix for 8157468?

Duplicated code was deleted, minor refactoring done, deleted not needed 
import.

WebRev: http://cr.openjdk.java.net/~kzhaldyb/webrevs/JDK-8157468/webrev.00/
CR: https://bugs.openjdk.java.net/browse/JDK-8157468

Thank you.

Regards, Kirill