docs: add docs for tests and mock

docs/advanced/testing.rst

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+Testing
+*******
+
+We use pytest for testing.
+
+You can run specific tests by specifying the file.::
+
+    $ pytest tests/test_twistmoe.py
+
+
+Mocks
+*****
+
+The tool provides a framework for easy mocking of providers.
+To use it, run the tool using the new provider. In this doc, we will use the example of twist.moe.::
+
+    $ anime -ll DEBUG dl 'shingeki no kyojin' --provider twist.moe --url -e 1
+    ....
+    2019-09-28 18:16:19 this anime_downloader.sites.helpers.request[521432] DEBUG HTML file temp_dir: /tmp/animedlszzxne7y
+    ....
+
+
+In the above output, we can see the temp directory created by the tool.
+Copy this temp directory to tests/test_sites and name it test_<your provider name>.
+
+After this you are ready to write the tests. The twist.moe test file is given below for reference.
+
+Remember to use the function :py:function:`configure_httpretty(<your provider name>)` to configure the mock before making any requests.
+
+.. code:: python
+
+    import pytest
+
+    from anime_downloader.sites.twistmoe import TwistMoe
+    from test_sites.site import configure_httpretty
+
+
+    @pytest.fixture
+    def anime():
+        return TwistMoe('https://twist.moe/a/shingeki-no-kyojin/first')
+
+    configure_httpretty('twistmoe')
+
+    def test_search():
+        ret = TwistMoe.search('shingeki no kyojin')
+        assert len(ret) == 5
+        assert ret[0].title == 'Shingeki no Kyojin'
+
+
+    def test_title(anime):
+        assert anime.title == 'shingeki-no-kyojin'
+
+
+    def test_length(anime):
+        assert len(anime) == 25
+
+
+    def test_streamurl(anime):
+        assert anime[0].source().stream_url == 'https://eu1.twist.moe/anime/attackontitan/[Coalgirls]_Shingeki_no_Kyojin_01_(1920x1080_Blu-ray_FLAC)_[AEF12794].mp4'