[R] Add test for new nixlibs.R

[assignUser]

Describe the enhancement requested

#38236 and previous PRs add new functionality to nixlibs.R which are mostly not covered in test-nixlibs.R.

Component(s)

R

[paleolimbot]

We need to fix #37792 (comment) while we're here!

[paleolimbot]

Also, a fun error from the other R build log:

Error : invalid version specification ‘integer(0).integer(0).integer(0).integer(0)’
⠏ |          0 | nixlibs                                                        *** Failed to find latest nightly for 8.0.0.9000

[assignUser]

Ah yeah for the tests we set the version to that... some gating required I guess ^^