feat: more RNTuple test files

[ariostas]

I added two more files to test RNTuple on uproot.

One of them contains std::atomic and std::bitset fields, which will be supported after scikit-hep/uproot5#1223.

The other file contains uncommon cases that the RNTuple spec supports, but we're still not sure if we will support them on uproot. This file includes a field consisting of an empty struct and another field consisting of an std::variant that is not always in a valid state.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.18%. Comparing base (9ed1610) to head (a266ce4).
Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
+ Coverage   69.78%   73.18%   +3.40%     
==========================================
  Files           3        3              
  Lines         139      138       -1     
==========================================
+ Hits           97      101       +4     
+ Misses         42       37       -5     

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