[lldb][ARM] Print mismatched registers in emulation tests

Also correct the test failed message. It implies that what
it's done is compare the 'before' and 'ater' states from the
test input.

Except that that's the whole point of the test, that the state changes.
It should tell you that it compared the result of the emulation to the
'after'.

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D130464