add test for npm.installed/removed

This is a simple test that would have caught the breakage I fixed in https://github.com/saltstack/salt/pull/13033. It will fail without that being merged, so this will need to be rebased before merging.

Also, it will be skipped if npm is not installed, so we'll need a way to ensure that it is installed before the test suite is run (or at least before this test is run). @s0undt3ch, what is the best way of doing this?