[clang] [analyzer][NFC] Reorganize bstring.cpp tests (PR #188709)

[Balázs Benics via cfe-commits]

steakhal wrote:

> LGTM, I like this approach. (Disclaimer: I didn't hunt for typos and subtle errors.)
> 

Let me have a look then before merging.

https://github.com/llvm/llvm-project/pull/188709