[llvm] Add special case for unsupported test (PR #78858)

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

Could you add `[doc]` to the start of the commit title, please? The way the title is currently it makes it look like there's some functional change to the testing infrastructure in this patch.

https://github.com/llvm/llvm-project/pull/78858