Declare static constants before functions

See the Google C++ Style Guide:
https://google.github.io/styleguide/cppguide.html#Declaration_Order

Suggested by James Zern.
diff --git a/tests/gtest/avifprogressivetest.cc b/tests/gtest/avifprogressivetest.cc
index 4ae55a6..313b401 100644
--- a/tests/gtest/avifprogressivetest.cc
+++ b/tests/gtest/avifprogressivetest.cc
@@ -10,6 +10,8 @@
 
 class ProgressiveTest : public testing::Test {
  protected:
+  static constexpr uint32_t kImageSize = 256;
+
   void SetUp() override {
     if (avifCodecName(AVIF_CODEC_CHOICE_AOM, AVIF_CODEC_FLAG_CAN_ENCODE) ==
         nullptr) {
@@ -49,8 +51,6 @@
     // quality layer is more similar.
   }
 
-  static constexpr uint32_t kImageSize = 256;
-
   testutil::AvifEncoderPtr encoder_{avifEncoderCreate(), avifEncoderDestroy};
   testutil::AvifDecoderPtr decoder_{avifDecoderCreate(), avifDecoderDestroy};