Support Empty Globs for `#[test_resources(...)]`

[almann]

We have some test code in which a glob currently yields no results, but if the test vector suite ever adds a file with that pattern, it would be nice to have it just work.

E.g. our tests do this to remind ourselves about this future case:

#[test_resources("ion-tests/iontestdata/good/non-equivs/**/*.ion")]
// no binary files exist and the macro doesn't like empty globs...
//#[test_resources("ion-tests/iontestdata/good/non-equivs/**/*.10n")]
fn non_equivs(file_name: &str) {

It would be nice to allow the empty glob to work, or at least have a parameter to allow it.