[backport#17291] tests: Add fuzzing harness for ISO-8601 related functions

Summary:
595cc9bcafb16709c0dd4a36d5921feb009ce430 docs: Add undefined to --with-sanitizers=fuzzer,address (practicalswift)
d5dbb4898c437d3d2a5798d79c3986ba9e0b72f9 tests: Add fuzzing harness for ISO-8601 related functions (practicalswift)

Pull request description:

Add fuzzing harness for ISO-8601 related functions.

**Testing this PR**

Run:

```
$ CC=clang CXX=clang++ ./configure --enable-fuzz \
      --with-sanitizers=address,fuzzer,undefined
$ make
$ src/test/fuzz/parse_iso8601
…
```

Depends on D6896

Backport of Core PR17291

Test Plan:

cmake -GNinja .. -DENABLE_SANITIZERS="address;fuzzer" -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++
ninja bitcoin-fuzzers
./src/test/fuzz/parse_iso8601

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6898