[ntuple] Add tests for forward compatibility

[silverweed]

This Pull request:

depends on https://github.com/root-project/root/pull/15553. The interesting file to look at is ntuple_compat.cxx.

Adding some RNTuple compat tests where we handcraft an on-disk representation for a RNTuple (and its wrapping TKey) and we try to read it back to verify it works. The reason to do this is that we can then mess around with the binary representation to simulate both a corrupted payload and a future version of RNTuple, which allows to test forward compatibility.

Checklist:

• [x] tested changes locally
• [ ] updated the docs (if necessary)

[github-actions[bot]]

Test Results

    11 files      11 suites   2d 11h 15m 49s :stopwatch:  2 645 tests  2 645 :white_check_mark: 0 :zzz: 0 :x: 27 455 runs  27 455 :white_check_mark: 0 :zzz: 0 :x:

Results for commit c7d052a0.

:recycle: This comment has been updated with latest results.