internal/fs: clean up setupInaccessibleDir helper
internal/fs: clean up setupInaccessibleDir helper

Updates #632

setupInaccessibleDir returns an error which must be checked by the
caller, which always invoked t.Fatal on error.

This change refactors setupInaccessibleDir to call t.Fatal internally
and handle any clean up if necessary.

ibrasho-forks/dep,golang/dep,ibrasho-forks/dep,golang/dep,golang/dep