fix(test): Remove uses of t.FailNow in goroutines (#1978)

It is documented as illegal to use t.FailNow in a goroutine.
Uses of require assertions call t.FailNow. This may have led to
inconsistent test results as some assertions were subject to
undefined behavior. Note that it is still allowed to call
assert assertions in goroutines.

See: https://pkg.go.dev/testing#T.FailNow

internal/servers/controller/handler_test.go

@@ -250,13 +250,18 @@ func TestCallbackInterceptor(t *testing.T) {
 		Handler: wrapHandlerWithCallbackInterceptor(noopHandler, nil),
 	}
 
+	// Use error channel so that we can use test assertions on the returned error.
+	// It is illegal to call `t.FailNow()` from a goroutine.
+	// https://pkg.go.dev/testing#T.FailNow
+	errChan := make(chan error)
 	go func() {
-		if err := server.Serve(listener); err != nil {
-			if err != http.ErrServerClosed {
-				require.NoError(t, err)
-			}
-		}
+		errChan <- server.Serve(listener)
 	}()
+	t.Cleanup(func() {
+		if err := <-errChan; err != http.ErrServerClosed {
+			require.NoError(t, err)
+		}
+	})
 
 	testCases := []struct {
 		name     string