Learn Go with Tests adds a chapter on testing/synctest

Chris James has added a new chapter to Learn Go with Tests covering testing/synctest, revisiting how to write time-dependent tests without the usual sleep-and-pray dance. Alongside it, a field report on the green tea garbage collector walks through a Kafka consumer where the new default did nothing for throughput and instead exposed partition rebalancing as the real bottleneck, which is a useful counterweight to the release-notes narrative.