cmd/gomobile: remove temporary files used in tests

Change-Id: I792edeed6bf8309084281eebcdba7d4b6c6a0818
Reviewed-on: https://go-review.googlesource.com/4181
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

cmd/gomobile/writer_test.go

@@ -29,12 +29,14 @@ func TestWriter(t *testing.T) {
 		t.Fatal(err)
 	}
 	f.Close()
+	defer os.Remove(f.Name())
 	apkPath := f.Name() + ".apk"
 
 	f, err = os.Create(apkPath)
 	if err != nil {
 		t.Fatal(err)
 	}
+	defer os.Remove(apkPath)
 
 	apkw := NewWriter(f, privKey)