Additional tests for attributes

[eddelbuettel]

This pull request is a bit of an experiment in that I asked Claude Sonnet 4.5 (via the Google Antigravity IDE, another Code clone) to examine existing #nocov tags and fill in some tests. And so it did which we can now look at in more detail.

It is all fairly reasonable if pedestrian (i.e. checking on Windows for permitted filenames etc) but does exactly what it was asked: increase coverage of tests across the codebase by a bit. If we like this we can repeat as this touched only one C++ and one R file so far (for attributes, in this case).

I did one minimal round of postprocessing so far. In one case we still need the #nocov: the windows-specific code will never run under Linux coverage analysis. And one expected error does not materialize as the compilation fails (cannot have two functions in cppFunction() call).

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog