s390x regtest: Reduce test output for vector test cases

s390x regtest: Reduce test output for vector test cases

The s390x test cases `vector', `vector_integer', `vector_float' and
`vector_string' generate a lot of unnecessary output, because for each
tested instruction they usually dump more registers than the instruction
modifies.

Without changing the test approach itself and without losing information,
the output can still be reduced significantly by comparing the register
states before and after the tested instruction and then only dumping the
changed parts.