Unify CopySwap and CopyMemory tests

As a temporary shortcut for JDK-8149159 I chose to copy the jdk/test/jdk/internal/misc/Unsafe/CopySwap.java test (which tests Unsafe.copySwapMemory) and just adapt it for testing Unsafe.copyMemory. However, a lot of the test code is the exact same, and having two copies of it is for obvious reasons not ideal.

The CopySwap and CopyMemory tests should be unified and to the extent possible the code should be shared.