misc/cgo/errors: fix TestPointerChecks when GO111MODULE=on

Change-Id: I4e9e46f03c9c43df1d0c6995f3baedd2e1a04c6a
Reviewed-on: https://go-review.googlesource.com/c/go/+/183985
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

misc/cgo/errors/ptr_test.go

@@ -466,6 +466,9 @@ func buildPtrTests(t *testing.T) (dir, exe string) {
 	if err := os.MkdirAll(src, 0777); err != nil {
 		t.Fatal(err)
 	}
+	if err := ioutil.WriteFile(filepath.Join(src, "go.mod"), []byte("module ptrtest"), 0666); err != nil {
+		t.Fatal(err)
+	}
 
 	// Prepare two cgo inputs: one for standard cgo and one for //export cgo.
 	// (The latter cannot have C definitions, only declarations.)