[CMAKE] Build test_bitcoin_fuzzy
[CMAKE] Build test_bitcoin_fuzzy

Summary:
Build the fuzzer glue executable.
Excluded from default build, as it makes no sense to build it without
instrumentation.

Test Plan:
WARNING: the test plan will require you to clear your ccache once done.

Needs D4578 to pass:
  cmake -GNinja .. -DCMAKE_C_COMPILER=afl-clang-fast \
    -DCMAKE_CXX_COMPILER=afl-clang-fast++
  ninja test_bitcoin_fuzzy
Follow the instructions from `fuzzing.md` starting at section `Preparing
fuzzing` and check it runs as expected.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4579

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