arcyd/test_shell: fail if 8000 already in use

Avoid the surprising result of 'poke' not creating a review by checking
that the 'git snoop web server' port is not already taken.

Print out the tail of the 'git snoop web server' logs to make it more
obvious in the case where something went wrong.

Test Plan:

$ python -m SimpleHTTPServer 8000 &
$ ./testbed/arcyd/test_shell.sh
.. immediately fail with new error message ..
$ killall python
$ ./testbed/arcyd/test_shell.sh
.. normal operation, tail of log printed at end ..