gtest/avif_fuzztest_helpers.cc is compiled once for each fuzztest target

[wantehchang]

In tests/CMakeLists.txt, the add_avif_fuzztest macro is defned as follows:

    macro(add_avif_fuzztest TEST_NAME)
        add_executable(${TEST_NAME} gtest/${TEST_NAME}.cc gtest/avif_fuzztest_helpers.cc ${ARGN})
        ...

I think this means gtest/avif_fuzztest_helpers.cc is compiled once for each fuzztest target. We should move it to an object or static library so that it is compiled only once for all fuzztest targets.

[vrabaud]

I had not done so because this requires calling link_fuzztest on the object library and that function is totally opaque (e.g. it might just work on a binary). Luckily, all it does is calling target_link_libraries, cf https://github.com/google/fuzztest/blob/02daa2ab69f514109c8831d72a13ffab49f6f375/cmake/AddFuzzTest.cmake#L1

Feel free to approve #2417 to get a solution that works until link_fuzztest does not change behavior.

[vrabaud]

Fixed by #2417