Skip some tests in avifchangesettingtest using aom

GTEST_SKIP the tests using codecChoice = AVIF_CODEC_CHOICE_AOM.
diff --git a/tests/gtest/avifchangesettingtest.cc b/tests/gtest/avifchangesettingtest.cc
index effd7e7..159787a 100644
--- a/tests/gtest/avifchangesettingtest.cc
+++ b/tests/gtest/avifchangesettingtest.cc
@@ -161,6 +161,11 @@
 }
 
 TEST(ChangeSettingTest, UnchangeableImageColorRange) {
+  if (avifCodecName(AVIF_CODEC_CHOICE_AOM, AVIF_CODEC_FLAG_CAN_ENCODE) ==
+      nullptr) {
+    GTEST_SKIP() << "Codec unavailable, skip test.";
+  }
+
   constexpr uint32_t kImageSize = 512;
   testutil::AvifImagePtr image = testutil::CreateImage(
       kImageSize, kImageSize, /*depth=*/8, AVIF_PIXEL_FORMAT_YUV420,
@@ -193,6 +198,11 @@
 }
 
 TEST(ChangeSettingTest, UnchangeableImageChromaSamplePosition) {
+  if (avifCodecName(AVIF_CODEC_CHOICE_AOM, AVIF_CODEC_FLAG_CAN_ENCODE) ==
+      nullptr) {
+    GTEST_SKIP() << "Codec unavailable, skip test.";
+  }
+
   constexpr uint32_t kImageSize = 512;
   testutil::AvifImagePtr image = testutil::CreateImage(
       kImageSize, kImageSize, /*depth=*/8, AVIF_PIXEL_FORMAT_YUV420,