to know that "which" refer to "/usr/local/bin/node" isn't something really important.
1- it can be any other previous version installed, a link, or anything else
2- binary can install in /usr/bin without any major problem (in term of functional)
2- if we refactor cookbook later (by adding a dir in PATH for example) this test fail... that can be bad :)

IMPOV, tests must verified that node is installed, accessible and at version user want.
Anything else is redundant with recipe.