Add a hook to customise client connection pipeline in tests

[aryan-25]

Motivation

There are two tests in HTTP3DatagramTests which set up a client-server connection and test for reading/writing unreliable datagrams on the connection.

The client handler in these tests immediately starts writing datagrams without waiting for the server's SETTINGS frame to arrive. If the write happens to occur before the SETTINGS arrive, the underlying swift-nio-http3 stack drops the write. This results in the test hanging until the connection idle timeout closes the connection, at which point all #expect/#require assertions fail.

We have run into this race in some CI runs over the past few days.

Modifications

• Added a new configuration option named additionalConnectionChannelInitializer to TestHelpers.ClientConfiguration. This can be used to add additional handlers to the connection pipeline in tests.
• Updated the affected tests to:
  1. Add a handler to the connection channel which succeeds a provided promise when the server has confirmed that it supports unreliable datagrams (through the SETTINGS frame), and;
  2. Wait for the result of the promise before writing unreliable datagrams.

Result

The affected tests in HTTP3DatagramTests are now more reliable.