tests: Clear remove/remock approach for ACCESS_TOKEN_URL endpoint.
tests: Clear remove/remock approach for ACCESS_TOKEN_URL endpoint.

ACCESS_TOKEN_URL works a different for apple authentication, so,
we removed and remocked the ACCESS_TOKEN_URL mock in
`register_extra_endpoints` override of apple auth test class.
It is cleaner to have it as generic feature of `social_auth_test`.

So, this commit adds a function that returns token_data_dict that
we had earlier and is called in the ACCESS_TOKEN_URL mock.
This function is overriden in apple auth test class to generate
payload of the format that apple auth expects.

Thanks to Mateusz Mandera for the simple idea.

Co-authored-by: Mateusz Mandera <f7f32f9a6b47b7a73ccf8377c70c7d8df4eff3b6@zulip.com>

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