Compilation fail where c_char = u8

[itsjunetime]

Describe the bug
Arrow integration tests assume that c_char = i8. While this is true on most platforms, there are many where it is not true (such as arm linux and risc-v linux). This causes a compilation failure in arrow-integration-testing

To Reproduce
Compile on a target where c_char = u8 (e.g. aarch64-unknown-linux-gnu)

Expected behavior
It should compile fine

Additional context
I'll file a PR to fix this.

[alamb]

label_issue.py automatically added labels {'arrow'} from #6572