[gdb/testsuite] Handle REMOTE_HOST_USERNAME in local-remote-host

Handle REMOTE_HOST_USERNAME in local-remote-host, similar to how that's done for
REMOTE_TARGET_USERNAME in remote-gdbserver-on-localhost.

This helps to keep the home dir clean.

Since the setup makes $build/gdb/testsuite on build unreadable for the remote
host, we run into permission problems for GDB and the data-directory, so fix
this (as was done for gdbserver in gdbserver-base.exp) using file normalize.

Tested on x86_64-linux.