Merge #17777: tests: Add fuzzing harness for DecodeHexTx(…)

3f95fb0 build: Sort fuzzing harnesses to avoid future merge conflicts (practicalswift)
bcad014 tests: Add fuzzing harness for DecodeHexTx(...) (practicalswift)

Pull request description:

  Add fuzzing harness for `DecodeHexTx(…)`.

  To test this PR:

  ```
  $ make distclean
  $ ./autogen.sh
  $ CC=clang CXX=clang++ ./configure --enable-fuzz \
        --with-sanitizers=address,fuzzer,undefined
  $ make
  $ src/test/fuzz/decode_tx
  …
  ```

ACKs for top commit:
  jonatack:
    ACK 3f95fb0

Tree-SHA512: 0f476d0cc26f1e03812664373118754042074bdab6c1e3a57c721f863feb82ca2986cceeaceb03192d893b9aa1d4ad8a5fb4c74824b9547fd8567805931a9ebd

Author: laanwj