These tests used to log the error message and return plain bool mainly
because at the time they we written, we did not have a nice way to
assert on llvm::Error values. That is no longer true, so replace this
pattern with a more idiomatic approach.
As a part of this patch, I also move the formatting of
GDBRemoteCommunication::PacketResult values out of the test code, as
that can be useful elsewhere.