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

[Thurston Dang via llvm-commits]

thurstond 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?

Looks like this patch did not solve the QEMU bot issues: https://lab.llvm.org/buildbot/#/builders/139/builds/5552/steps/30/logs/stdio

"A bug? In _QEMU_ code?
It's more likely than you think.
FREE PC EMULATOR!"

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