Remove flakiness of google chat tests (#40592)

pkg/services/ngalert/notifier/channels/testing.go

@@ -0,0 +1,23 @@
+package channels
+
+import "time"
+
+// mockTimeNow replaces function timeNow to return constant time.
+// It returns a function that resets the variable back to its original value.
+// This allows usage of this function with defer:
+// func Test (t *testing.T) {
+//    now := time.Now()
+//    defer mockTimeNow(now)()
+//    ...
+// }
+func mockTimeNow(constTime time.Time) func() {
+	timeNow = func() time.Time {
+		return constTime
+	}
+	return resetTimeNow
+}
+
+// resetTimeNow resets the global variable timeNow to the default value, which is time.Now
+func resetTimeNow() {
+	timeNow = time.Now
+}