8784: Fix test for latest firefox.

Given "http://host:99999/path", Firefox 54 returns an error
"SyntaxError: The URI is malformed" instead of trying to connect to
port 99999 and returning a network error. Changing 99999 to 0 produces
the try-and-fail behavior we want to test.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curoverse.com>