Increase coverage of CIRCL packages

Test coverage is in average 78%. Some packages need more test coverage.
Good to have unit tests that cover missing areas of code.

## Report from codecov.

Files | Tracked lines | Covered | Partial | Missed | Coverage %
-- | -- | -- | -- | -- | --
abe | 2499 | 1759 | 215 | 525 | 70.39%
blindsign | 353 | 244 | 24 | 85 | 69.12%
dh | 3441 | 2241 | 24 | 1176 | 65.13%
ecc | 5930 | 5692 | 24 | 214 | 95.99%
expander | 85 | 79 | 3 | 3 | 92.94%
group | 518 | 431 | 10 | 77 | 83.20%
hpke | 838 | 574 | 60 | 204 | 68.50%
internal | 184 | 137 | 3 | 44 | 74.46%
kem | 8 | 8 | 0 | 0 | 100.00%
math | 1082 | 1046 | 7 | 29 | 96.67%
oprf | 373 | 301 | 23 | 49 | 80.70%
ot | 145 | 100 | 19 | 26 | 68.97%
pke | 1344 | 1107 | 22 | 215 | 82.37%
pki | 120 | 71 | 17 | 32 | 59.17%
sign | 8343 | 5894 | 188 | 2261 | 70.65%
simd | 60 | 50 | 4 | 6 | 83.33%
tss | 460 | 372 | 29 | 59 | 80.87%
xof | 19 | 19 | 0 | 0 | 100.00%
zk | 247 | 174 | 27 | 46 | 70.45%