Test and simplify Goby's lib path assignment

[st0012]

During VM initialization (in vm/vm.go), Goby will find and store a path (called libPath) for loading Goby's standard libraries that are written in Goby code.

Currently, there are some improvements need to be done on the mechanism, e.g. use brew command to find goby's installation path. But because it doesn't have enough test coverage yet, we need to first add some integration test cases for it.