[cleanup] Skip test classes as a whole
[cleanup] Skip test classes as a whole

- Rename IndexPageTestCase to BS4TestCase
- use setupClass in BS4TestCase to verify bs4 module is available;
  raise SkipTest if not to skip the test class as a whole instead of
  skipping each test method of the class. SkipTest exception is necessary
  because skipTest method does not work for Python 2 inside setUpClass
  class method.

- Derive all test classes from BS4TestCase which need bs4
  and remove require_modules decorator

Change-Id: Iad96aa8875a109dc79bee08e36e2ec01786342a4

wikimedia/pywikibot-core,wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core