Fix for allpairs test failures.

AllPairs tests fail because it specifies the same file twice as inputs. This means, the worker will link the file twice from its cache to the task sandbox. The second time it tries to link, it fails since the file already exists in the sandbox. And the worker disconnects as a result.

This shouldn't be treated as a failure because this is never WQ's fault. Fix 13bb17d corrects this.

Fix 446558e is related. It checks for the return values of specify_file/buffer in allpairs so that the tests fail if there are errors in the file specification.

@dthain, @btovar