Add tests for `_decide_nvjitlink_or_driver()`

[leofang]

[rparolin]

@rwgk @leofang Do we still want to this to be included in the beta 9 release?

[rwgk]

Definitely good to have, but the cost / benefit is high short term.

Hermetic, meaningful tests for this are tricky. Refactoring is probably most practical.

[rparolin]

@rwgk : I'll bump it to next release.