On macOS, we can do "go build", can exec, and have the source
tree available, so we can enable more tests.
Skip ones that don't work. Most of them are due to that it
requires external linking (for now) and some tests don't work
with external linking (e.g. runtime deadlock detection). For
them, helper functions CanInternalLink/MustInternalLink are
introduced. I still want to have internal linking implemented,
but it is still a good idea to identify which tests don't work
with external linking.