sage: disable test timeout

I already did that once in 359bf7f1.
That change mysteriously got lost somehow (presumably in some merge
commit).

Nix has its own timeout settings, so there is no risk in running
forever. At the same time, some tests can exceed the default timeout
(30minutes per file for --long tests) when run on many weak cores (like
the aarch64 community builder or some hydra builders).