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/apitestlib.go

@@ -1093,12 +1093,6 @@ func CheckErrorMessage(tb testing.TB, err error, message string) {
 	require.Equalf(tb, message, appError.Message, "incorrect error message, actual: %s, expected: %s", appError.Id, message)
 }
 
-func CheckStartsWith(tb testing.TB, value, prefix, message string) {
-	tb.Helper()
-
-	require.True(tb, strings.HasPrefix(value, prefix), message, value)
-}
-
 // Similar to s3.New() but allows initialization of signature v2 or signature v4 client.
 // If signV2 input is false, function always returns signature v4.
 //