test/common.py: fix race condition waiting for the helper connection

test/common.py: fix race condition waiting for the helper connection

The wait_for_connections() method, which is used at the start of
test_system to make sure that all the clients are connected to all the
servers, did not also wait for clients to be connected to their Helpers.
Every once in a while, the helper connection would take a bit longer,
and then
test_system.SystemTest.test_filesystem._test_web._got_welcome_helper
would fail, because we'd check for a helper connection before it was
ready.

The fix is to modify wait_for_connections's polling predicate to look
for helper connections (if configured) as well as the regular
introducer- and server- connections.

Tested by temporarily adding a large (30s) delay to the connectTo() call
in Uploader.startService, simulating a long helper
connection-establishment delay. This makes the test fail consistently.
Then I fixed wait_for_connections(), and the test passed (slowly). Then
I removed the delay.

Closes #1467