fuzz-tests: Add a test for the gossipd-connectd interface

[Chand-ra]

connectd_req() in gossipd/gossipd.c is responsible for handling gossip messages from peers handed to it by connectd. Add a stateful test simulating its behaviour.

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.

CC: @morehouse

[Chand-ra]

The test results in the following LeakSanitizer error when run on its corpus:

==116428==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 40 byte(s) in 1 object(s) allocated from:
    #0 0x619eb51001f3 in malloc (/home/chandra/lightning/tests/fuzz/fuzz-gossipd-connectd+0x2da1f3) (BuildId: 34398486c14ff41d79a297932e621cb4f6161418)
    #1 0x619eb54deb3d in split_node /home/chandra/lightning/ccan/ccan/intmap/intmap.c:67:9
    #2 0x619eb54de8c2 in intmap_add_ /home/chandra/lightning/ccan/ccan/intmap/intmap.c:113:9
    #3 0x619eb53122d1 in map_add /home/chandra/lightning/gossipd/gossmap_manage.c:176:6
    #4 0x619eb530f8c6 in gossmap_manage_channel_announcement /home/chandra/lightning/gossipd/gossmap_manage.c:640:8
    #5 0x619eb53b2101 in handle_recv_gossip /home/chandra/lightning/tests/fuzz/../../gossipd/gossipd.c:205:12
    #6 0x619eb53a93b2 in run /home/chandra/lightning/tests/fuzz/fuzz-gossipd-connectd.c:543:4
    #7 0x619eb513e8f8 in LLVMFuzzerTestOneInput /home/chandra/lightning/tests/fuzz/libfuzz.c:25:2
    #8 0x619eb504c0c4 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/chandra/lightning/tests/fuzz/fuzz-gossipd-connectd+0x2260c4) (BuildId: 34398486c14ff41d79a297932e621cb4f6161418)
    #9 0x619eb504b7b9 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*) (/home/chandra/lightning/tests/fuzz/fuzz-gossipd-connectd+0x2257b9) (BuildId: 34398486c14ff41d79a297932e621cb4f6161418)
    #10 0x619eb504d3d6 in fuzzer::Fuzzer::ReadAndExecuteSeedCorpora(std::vector<fuzzer::SizedFile, std::allocator<fuzzer::SizedFile>>&) (/home/chandra/lightning/tests/fuzz/fuzz-gossipd-connectd+0x2273d6) (BuildId: 34398486c14ff41d79a297932e621cb4f6161418)
    #11 0x619eb504d8e7 in fuzzer::Fuzzer::Loop(std::vector<fuzzer::SizedFile, std::allocator<fuzzer::SizedFile>>&) (/home/chandra/lightning/tests/fuzz/fuzz-gossipd-connectd+0x2278e7) (BuildId: 34398486c14ff41d79a297932e621cb4f6161418)
    #12 0x619eb503addf in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/chandra/lightning/tests/fuzz/fuzz-gossipd-connectd+0x214ddf) (BuildId: 34398486c14ff41d79a297932e621cb4f6161418)
    #13 0x619eb5065466 in main (/home/chandra/lightning/tests/fuzz/fuzz-gossipd-connectd+0x23f466) (BuildId: 34398486c14ff41d79a297932e621cb4f6161418)
    #14 0x7aed7c22a1c9 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #15 0x7aed7c22a28a in __libc_start_main csu/../csu/libc-start.c:360:3
    #16 0x619eb502fdc4 in _start (/home/chandra/lightning/tests/fuzz/fuzz-gossipd-connectd+0x209dc4) (BuildId: 34398486c14ff41d79a297932e621cb4f6161418)

SUMMARY: AddressSanitizer: 40 byte(s) leaked in 1 allocation(s).

INFO: a leak has been found in the initial corpus.