[compiler-rt] [asan] Flush stderr in test (PR #114084)

[Florian Mayer via llvm-commits]

fmayer wrote:

> > Shouldn't stderr be unbuffered?
> 
> Hmm, that's a good point. Should I still merge to see if it works around the QEMU issue?

I'm not opposed. Could this be a qemu bug?

https://github.com/llvm/llvm-project/pull/114084