58d67f1cc068c3779e309dc8a82ce33158c3e5b2 tests: Add fuzzing harness for descriptor Span-parsing helpers (practicalswift)
Pull request description:
Add fuzzing harness for descriptor Span-parsing helpers (`spanparsing`).
As suggested by a fuzz testing enthusiast in https://github.com/bitcoin/bitcoin/pull/16887#issuecomment-540655816.
**Testing this PR**
Run:
```
$ CC=clang CXX=clang++ ./configure --enable-fuzz \
--with-sanitizers=address,fuzzer,undefined
$ make
$ src/test/fuzz/spanparsing
```Depends on D6879
Backport of Core PR17113