RFR(S): 8151233: Unify CopySwap and CopyMemory tests

[Mikael Vidstedt]

Please review the below change.

When doing the clean up of Unsafe I cheated and made a copy of the 
CopySwap test (used to test U.copySwapMemory) for testing U.copyMemory. 
Much of the code is the same, or at least similar enough, that 
duplication is unfortunate. This change moves the common code to a 
helper file CopyCommon.java, and makes both test files use that.

Bug: https://bugs.openjdk.java.net/browse/JDK-8151233
Webrev: http://cr.openjdk.java.net/~mikael/webrevs/8151233/webrev.00/webrev/

Cheers,
Mikael