go/loader: use Skipf in test
go/loader: use Skipf in test

(late response to comment in review of CL 18452)

Change-Id: I1fa80fd1c7fd22cec752c839d20a483e0732337c
Reviewed-on: https://go-review.googlesource.com/18454
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

cmars/tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,cmars/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,cmars/tools,cmars/tools,golang/tools,syreclabs/go-tools,golang/tools