Fixing partial warning for missing cheatcode

[msooseth]

Description

We actually didn't produce a partial execution trace when a cheatcode was not found. I now added a test that would have failed before.

It now says:

ghci> :main -p "cheatcode-nonexistent"
hevm
  Panic code tests via symbolic execution
    cheatcode-nonexistent:    WARNING: hevm was only able to partially explore the call prefix 0x7a9839c2 due to the following issue(s):
     - Cheat code not recognized
     program counter: 181
     selector: 0xcd09029c
OK (0.02s)

Checklist

• tested locally
• added automated tests
• updated the docs
• updated the changelog

[blishko]

LGTM!

[msooseth]

Thanks!