Add a mix-in class ExtraAssertions containing the following methods:
* assertHasAttr() and assertNotHasAttr()
* assertIsSubclass() and assertNotIsSubclass()
* assertStartsWith() and assertNotStartsWith()
* assertEndsWith() and assertNotEndsWith()
(cherry picked from commit c6a566e47b)
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
(cherry picked from commit 06cad77a5b)