Write unit tests

[joewallwork]

We have unit tests covering most of the matrix module, but the opt and utils modules are not covered at all.

See https://github.com/mesh-adaptation/opt_adapt/actions/runs/12223241146/job/34094524760 from #23:

Name                  Stmts   Miss  Cover
-----------------------------------------
opt_adapt/matrix.py      84     13    85%
opt_adapt/opt.py        333    333     0%
opt_adapt/utils.py        9      9     0%
-----------------------------------------
TOTAL                   426    355    17%