tests: Introduce `which` parameter in `get_channel_name_arg()` helper
tests: Introduce `which` parameter in `get_channel_name_arg()` helper

In preparation for introducing spawn tests using more than one channel,
we need a way to extract specific channel name args by distinctive
labels.

All existing `*_spawn()` tests are adapted to include the label in the
passed command line argument; and all `*_server()` helpers are updated
with the new argument in the `get_channel_name_arg()` calls.

servo/ipc-channel