KVM: s390: selftests: Split memop tests

KVM: s390: selftests: Split memop tests

Split success case/copy test from error test, making them independent.
This means they do not share state and are easier to understand.
Also, new test can be added in the same manner without affecting the old
ones. In order to make that simpler, introduce functionality for the
setup of commonly used variables.

Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Link: https://lore.kernel.org/r/20220308125841.3271721-2-scgl@linux.ibm.com
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>