Fix Errorf format string for error

Also go ahead and mark `checkOverrides` as a `t.Helper()`

Test: go test
Change-Id: I368aabf4d032c02773416f27affa12eb40e77b40

cc/testing.go

@@ -739,12 +739,13 @@ func AssertExcludeFromRecoverySnapshotIs(t *testing.T, ctx *android.TestContext,
 }
 
 func checkOverrides(t *testing.T, ctx *android.TestContext, singleton android.TestingSingleton, jsonPath string, expected []string) {
+	t.Helper()
 	out := singleton.MaybeOutput(jsonPath)
 	content := android.ContentFromFileRuleForTests(t, out)
 
 	var flags snapshotJsonFlags
 	if err := json.Unmarshal([]byte(content), &flags); err != nil {
-		t.Errorf("Error while unmarshalling json %q: %w", jsonPath, err)
+		t.Errorf("Error while unmarshalling json %q: %s", jsonPath, err.Error())
 		return
 	}