Pymain was not adding e.g. directory of executed file to sys.path, and
as the result if there were e.g. 2 files
dir/hello.py # imports world
running `gpython dir/hello.py` would fail to import world.
The case for interactive console was also failing to setup sys.argv as
empty, so it was containing ['/path/to/gpython']. It was also, contrary
to standard python, unconditionally showing '>>>' prompt even when stdin
was not a tty.
Fix all that and add a test to cover pymain functionality.