Skip NA test on RISCV64, it is non-portable

[mr-c]

Fixes: #485

Inspired by #485 (comment)

[TimTaylor]

Disclaimer - I've nothing to do with fs development but, FWIW, my comment assumed you were patching downstream in Debian with the understanding that R generally will be a little broken on riscv.

I'm not sure skipping the test upstream is the right way to go. Personally I'd want to see that failure (the formatting is broken on riscv and this shows that). If upstream did decide to skip it they will probably want to do it by the testthat skip mechanism so it is atleast flagged in their test suite for future consideration.

[gaborcsardi]

AFAIU this is not specific to Debian, so I am going to skip the test using testthat. Let me know if that's not the right solution.

[gaborcsardi]

Thank you!