tests/http_tests.py: Fix unittest.skipTest calls
tests/http_tests.py: Fix unittest.skipTest calls

'unittest.skipTest' does not exist. Replace it with `self.skipTest`.

Bug: T204204
Change-Id: I6a4e55cc2e38a0ba26d9e881c125e9feaee6b326

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