Fix the zeroization test

[poljar]

The test used some internal API to test if our zeroization works. We have now switched to the publicly available Ed25519 secret key type.

Some additional changes were needed to adjust for the different sizes of the secret key material.

[poljar]

The test fails, but CI did not notice this because the make file eats the error code.

Not sure why the test fails since it works locally.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.28%. Comparing base (c2efa95) to head (a007b3a).
⚠️ Report is 130 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #194   +/-   ##
=======================================
  Coverage   90.28%   90.28%           
=======================================
  Files          34       34           
  Lines        1905     1905           
=======================================
  Hits         1720     1720           
  Misses        185      185           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.