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