code: func (a *Assertions) Containsf(s interface{}, contains interface{}, msg string, args ...interface{}) {
  	if h, ok := a.t.(tHelper); ok {
  		h.Helper()
  	}
  	Containsf(a.t, s, contains, msg, args...)
  }
docs: // Containsf asserts that the specified string, list(array, slice...) or map contains the
  // specified substring or element.
  //
  //    a.Containsf("Hello World", "World", "error message %s", "formatted")
  //    a.Containsf(["Hello", "World"], "World", "error message %s", "formatted")
  //    a.Containsf({"Hello": "World"}, "Hello", "error message %s", "formatted")