Enforce zero-padded ordering of testcases and testgroups

[Matistjati]

Currently, one of the biggest footguns is that there's no way to notice locally that naming your groups quadratic, linear, etc will result in the testgroups not showing up in the desired order when displayed in an online judge.

I haven't thought of exactly what the check should be, but some examples of allowed group names:
g1, g2, ...
g01, ..., g10
1,2,...

Some examples of disallowed group names:
linear,quadratic
g1, ..., g10

We do not need to enforce this for sample in 2025-09, since there, file names are relative data, sample comes before secret.
Unclear if it's worth the hassle to enforce this for Legacy; I don't think anyone will actually bother to go through and update old problems to fix this. (see Kodsport/testdata_tools#30)

This should of course never be an error, but a warning at most.