go/loader: update stdlib test after changes to stdlib

The os/user.lookup function no longer exists so instead we use
os/user.current.  (This function also existed in go1.5.)

Change-Id: Ic66d80acc186289331f1023e0145370feab6a001
Reviewed-on: https://go-review.googlesource.com/21281
Reviewed-by: Robert Griesemer <gri@golang.org>

go/loader/stdlib_test.go

@@ -152,7 +152,7 @@ func TestCgoOption(t *testing.T) {
 		pkg, name, genericFile string
 	}{
 		{"net", "cgoLookupHost", "cgo_stub.go"},
-		{"os/user", "lookupId", "lookup_stubs.go"},
+		{"os/user", "current", "lookup_stubs.go"},
 	} {
 		ctxt := build.Default
 		for _, ctxt.CgoEnabled = range []bool{false, true} {