Introduce helper function to return a time.Time pointer

Replace the 84 time.Parse calls in 12 *_test.go files with a helper function similar to Ptr. This issue will simplify the required constructs of a time.Time struct pointer.

Follow up issue on !2333 (merged).

Suggestion

func mustParseTime(v string) *time.Time {
	t, err := time.Parse(time.Layout, v)
	if err != nil {
		panic(fmt.Sprintf("invalid time string %q: %v", v, err))
	}

	return &t
}

cc @fforster