[roottest] missed one override

[ferdymercury]

This Pull request:

Changes or fixes:

since it derives from TObject, it must override the ClassDef functions

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    20 files      20 suites   3d 6h 53m 26s ⏱️
 3 114 tests  3 109 ✅ 0 💤 5 ❌
60 691 runs  60 686 ✅ 0 💤 5 ❌

For more details on these failures, see this check.

Results for commit cbf252e.

[linev]

Thanks!