Channels/api4 testing improvements (#22938)

* api4/post_test: fix missing TearDown

* api4/plugin_test: dont test timeout, saving 120s

* api4/channel_test: dont try to delete town square

* api4/channel_test: check public channel names deterministically

* api4/file_test: fix darwin assertions on go files

* api4/notify_admin_test: fix expect/actual order

* api4/team_test: make TestGetAllTeams deterministic

* api4/plugin_test: avoid nested test helpers

* api4/post_test: avoid nested test helpers

* api4/websocket_test: externalize log buffer

* testlib/helper: unset common env

* linting issues

* simplify TestGetFileHeaders

* team_test: leverage ElementsMatch

server/channels/testlib/helper.go

@@ -58,6 +58,11 @@ func NewMainHelperWithOptions(options *HelperOptions) *MainHelper {
 		os.Unsetenv("MM_SQLSETTINGS_DATASOURCE")
 	}
 
+	// Unset environment variables commonly set for development that interfere with tests.
+	os.Unsetenv("MM_SERVICESETTINGS_SITEURL")
+	os.Unsetenv("MM_SERVICESETTINGS_LISTENADDRESS")
+	os.Unsetenv("MM_SERVICESETTINGS_ENABLEDEVELOPER")
+
 	var mainHelper MainHelper
 	flag.Parse()