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/api4/websocket_test.go

@@ -424,10 +424,14 @@ func TestWebSocketUpgrade(t *testing.T) {
 	th := Setup(t)
 	defer th.TearDown()
 
+	buffer := &mlog.Buffer{}
+	err := mlog.AddWriterTarget(th.TestLogger, buffer, true, mlog.StdAll...)
+	require.NoError(t, err)
+
 	url := fmt.Sprintf("http://localhost:%v", th.App.Srv().ListenAddr.Port) + model.APIURLSuffix + "/websocket"
 	resp, err := http.Get(url)
 	require.NoError(t, err)
 	require.Equal(t, resp.StatusCode, http.StatusBadRequest)
 	require.NoError(t, th.TestLogger.Flush())
-	testlib.AssertLog(t, th.LogBuffer, mlog.LvlDebug.Name, "Failed to upgrade websocket connection.")
+	testlib.AssertLog(t, buffer, mlog.LvlDebug.Name, "Failed to upgrade websocket connection.")
 }