Large files for testing

Hi,

We are currently in the process of converting the Math renderer from
JS to PHP. In this context, we deal with large (50Mb) files for
testing https://phabricator.wikimedia.org/T315223. While the test file
was part of the JS source code repo, we are unsure if this is also the
best practice for PHP code, as I don't remember having seen that in
other repositories such as core.
If you have experience with this, please let me know.

Thank you
physikerwelt