Adds unit tests for IsRunningInEE

This depends on !1417 (closed) to be merged first. Otherwise, a forced user authentication check would be executed in the init() method of internal/provider/testutil/helpers.go when importing the testutil package, so this wouldn't work as an unit test.

To also avoid actually dispatching the GetMetadata call, I've extracted the logic that needed to be tested to an auxiliary method called isEnterpriseInstance.