custom-scalars: fix bug in data download logic and add tests

[nfelt]

This fixes the custom scalars dashboard so the data download links work again. They have been failing on the backend with the following error due to an argument mismatch from when the experiment support was added:

File ".../python3.7/site-packages/tensorboard/plugins/custom_scalar/custom_scalars_plugin.py", line 126, in download_data_route
    body, mime_type = self.download_data_impl(run, tag, response_format)
TypeError: download_data_impl() missing 1 required positional argument: 'response_format'

To prevent regressions we port the download behavior HTTP-level tests from #3793 to the custom scalars dashboard. Also tested manually by launching tensorboard with test data and confirming the downloads now work as expected.

[wchargin]

Thanks! 🙏