KVM: s390: selftests: Add selftest for single-stepping

Test different variations of single-stepping into interrupts:

- SVC and PGM interrupts;
- Interrupts generated by ISKE;
- Interrupts generated by instructions emulated by KVM;
- Interrupts generated by instructions emulated by userspace.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-ID: <20230725143857.228626-7-iii@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
[frankja@de.igm.com: s/ASSERT_EQ/TEST_ASSERT_EQ/ because function was
renamed in the selftest printf series]
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>