We used to run "expect -c 'spawn ls'" for this in Binutils, but then we
thought expect test suite was enough as such a simple PTY test. However
expect test can fail due to some different reason, so add back a simple
test using Python pty module before building expect. Now we no longer
need to consider expect test critical (IIRC there was a report saying
one expect test failed for unknown reason but all other things OK).
