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

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gamesh411 wrote:

I have considered the following approach first:
https://github.com/llvm/llvm-project/commit/23514c5262e28f3cae352a0cfbe5c7de6b186a9e
That one increases test coverage at the cost of introducing some duplication in diagnostics expectations, as we have discussed in #186802.
The current PR makes the tests simpler (technically reducing coverage, but IMO, in practice, coverage stays the same: we only lose the non-header-guarded code's coverage from run-lines that *do* use those preprocessor symbols). So there is a tradeoff here.
I am not heavily invested in either solution.
@steakhal, if you think that direction is better than this, I'll just update this PR to go that way. 

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