misc/cgo/errors: fix tests in module mode

Updates #30228

Change-Id: I84bc705591bdb3da0106404b24353251939355b8
Reviewed-on: https://go-review.googlesource.com/c/163209
Reviewed-by: Jay Conrod <jayconrod@google.com>

misc/cgo/errors/errors_test.go

@@ -18,7 +18,7 @@ import (
 )
 
 func path(file string) string {
-	return filepath.Join("src", file)
+	return filepath.Join("testdata", file)
 }
 
 func check(t *testing.T, file string) {

misc/cgo/errors/ptr_test.go

@@ -444,8 +444,8 @@ func testOne(t *testing.T, pt ptrTest) {
 	}
 	defer os.RemoveAll(gopath)
 
-	src := filepath.Join(gopath, "src")
-	if err := os.Mkdir(src, 0777); err != nil {
+	src := filepath.Join(gopath, "src", "ptrtest")
+	if err := os.MkdirAll(src, 0777); err != nil {
 		t.Fatal(err)
 	}
 
@@ -490,6 +490,11 @@ func testOne(t *testing.T, pt ptrTest) {
 		}
 	}
 
+	gomod := fmt.Sprintf("module %s\n", filepath.Base(src))
+	if err := ioutil.WriteFile(filepath.Join(src, "go.mod"), []byte(gomod), 0666); err != nil {
+		t.Fatalf("writing go.mod: %v", err)
+	}
+
 	args := func(cmd *exec.Cmd) string {
 		return strings.Join(cmd.Args, " ")
 	}