[llvm] [test] Align behavior of interrupts.test on different platforms (PR #68556)

[Serge Pavlov via llvm-commits]

spavloff wrote:

Thank you for clarification.
In the new fix stderr is propagated to calling process so we can get actual stderr of `llvm-symbolizer`.
When llvm-symbolizer starts complaining on missed 'foo', we could replace
```
# RUN: count 0 < %t.err`
```
with
````
# RUN: FileCheck --input-file=%t.err %s
# CHECK: {{.*}} error: 'foo': {{[Nn]}}o such file or directory
# CHECK-NOT: {{.+}}
```
It should be enough to make the check you intended, no?

https://github.com/llvm/llvm-project/pull/68556