DMSReader: convert_pos_from_native coverage

[IAlibay]

Follow on from PR #3132

The convert_pos_from_native code isn't being covered, most likely because the tests always had dimensions set to None. We should try to add a test here.

See: https://github.com/MDAnalysis/mdanalysis/pull/3132/files#r628893346

[IAlibay]

Moving to 2.1.0 unless someone wants to work on this.

[orbeckst]

I'll add help wanted/GSOC starter label – remove again if it's too tricky.

(Also, if anyone else wants to procrastinate and write tests, please do ;-) )