This would be complex to implement (sorting isn't, per se, but proper implementation which consistently extends to Differential, Diffusion, Owners, arc, etc.) and this use case doesn't seem very strong (the behavior is obvious, there's a table of contents and a scrollbar, there's no particular reason not to review tests first, etc.).

If this is a major issue, consider putting tests in a directory called zzztests or similar. Notably, Facebook used a directory named zeusgodofthunder/ to hold subproject code for a long period of time after users complained that the previous, more accurate name was disrupting tab completion.