Check fread return value in datarate_test.cc

This is to fix
https://ci.chromium.org/ui/p/chromium/builders/try/linux-rel/2550106/overview

```
[75930/86333] CXX obj/third_party/libaom/test_libaom/datarate_test.o
../../third_party/llvm-build/Release+Asserts/bin/clang++ -MMD -MF obj/third_party/libaom/test_libaom/datarate_test.o.d -DCHROMIUM -DLIBAOM_TES...(too long)
../../third_party/libaom/source/libaom/test/datarate_test.cc:55:5: error: ignoring return value of function declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
   55 |     fread(img_input_->img_data, raw_size_, 1, input_file_);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Bug: 482138436
Change-Id: I18bef581d74e458d862b5520f6a3cae801d80c0b

test/datarate_test.cc

diff --git a/test/datarate_test.cc b/test/datarate_test.cc
index 9f3fff8..c777d4a 100644
--- a/test/datarate_test.cc
+++ b/test/datarate_test.cc
@@ -52,7 +52,7 @@
   void FillFrame() override {
     // Read frame from input_file and scale up.
     ASSERT_NE(input_file_, nullptr);
-    fread(img_input_->img_data, raw_size_, 1, input_file_);
+    ASSERT_EQ(fread(img_input_->img_data, raw_size_, 1, input_file_), 1);
     libyuv::I420Scale(
         img_input_->planes[AOM_PLANE_Y], img_input_->stride[AOM_PLANE_Y],
         img_input_->planes[AOM_PLANE_U], img_input_->stride[AOM_PLANE_U],