KVM: s390: selftests: Use TAP interface in the sync_regs test

KVM: s390: selftests: Use TAP interface in the sync_regs test

The sync_regs test currently does not have any output (unless one
of the TEST_ASSERT statement fails), so it's hard to say for a user
whether a certain new sub-test has been included in the binary or
not. Let's make this a little bit more user-friendly and include
some TAP output via the kselftests.h interface.
To be able to distinguish the different sub-tests more easily, we
also break up the huge main() function here in more fine grained
parts.

Acked-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20220531101554.36844-3-thuth@redhat.com
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>