test_system.py: clean up control flow, reduce use of stall()

test_system.py: clean up control flow, reduce use of stall()

The _upload_resumable() test interrupts a Helper upload partway
through (by shutting down the Helper), then restarts the Helper and
resumes the upload. The control flow is kind of tricky: to do anything
"partway through" requires adding a hook to the Uploadable. The previous
flow depended upon a (fragile) call to self.stall(), which waits a fixed
number of seconds.

This removes one of those stall() calls (the remainder is in
test/common.py and I'll try removing it in a subsequent revision). It
also removes some now-redundant wait_for_connections() calls, since
bounce_client() doesn't fire its Deferred until the client has finished
coming back up (and uses wait_for_connections() internally to do so).