It would be nice if R CMD check ran any file in the tests directory
that has "one of the extensions .R, .S, .q, .r, or .s" - i.e. it
should match the files processed in the R directory.
Whereas what it is documented in 'Writing R Extensions' is to use .R or
.Rin files. This leaves the other extensions available for other purposes
-- the difference is that there should be no other files in the R directory,
but often will be in the tests directory.
I have some sympathy for .r (which we allow for demos) given the prevalence
of case-insensitive file systems, but why would it be 'nice' to have the
others? And what would the output file be called? Should .S be processed
to .Sout and matched to a .Sout.save file? This would seem to introduce
more confusion that it might remove.