[RF] compiler-dependent RooNaNPacker tests

[egpbos]

Like the tests added in https://github.com/root-project/root/pull/15499/commits/e00aded416c4d8b0c364035353dfb529602fd0e1, these are also sanity checks meant in part as code-as-documentation for users to see how things work.

It turns out in this case that GCC on the one hand and Clang and MSVC on the other hand have two different solutions for binary arithmetic operations on two NaNs. In both cases, one of the two NaNs is returned, but in one case it's the first, in the other is it's the second.

The tests added in this commit will hopefully warn RooNaNPacker users of this behavior.

[github-actions[bot]]

Test Results

    18 files      18 suites   4d 6h 52m 21s ⏱️  2 678 tests  2 676 ✅ 0 💤  2 ❌ 46 484 runs  46 474 ✅ 0 💤 10 ❌

For more details on these failures, see this check.

Results for commit 97d322ce.

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