[llvm] Add test for dwarf verification JSON output (PR #124936)

[via llvm-commits]

youngdfb wrote:

> > A follow up diff will modify the JSON and this ensures we can verify.
> 
> @youngdfb does this mean this test will fail on check-in? Or do you intend to extend this test after your future patch

No this is simply testing the existing JSON structure.  So it passes now.  A change would break this test, though, which is good.

https://github.com/llvm/llvm-project/pull/124936