When building with gcc I get this warning:
```
<...>TestBreakpointSetCallback.cpp:58:25: warning: cast from type ‘const char*’ to type ‘void*’ casts away qualifiers [-Wcast-qual]
58 | void *baton = (void *)"hello";
| ^~~~~~~
```
Use the address of a mutable global variable instead. All we care about
is that the address passed as the baton is the same one we get later.
