Refactor invalid url tests

test_batoto:TestBatoto.(test_invalid_chapter, test_invalid_url and
test_irrelevant_url) merged into test_batoto:TestBatoto.test_invalid_urls
89% coverage.
Refactor chapter page tests

test_batoto:TestBatoto.(test_get_chapter_pages & test_chapter_language)
merged into test_batoto:TestBatoto.test_get_chapter_pages. 89% coverage.