[test] Align behavior of interrupts.test on different platforms

The test llvm/Support/interrupts.test behaves differently on Linux and
Windows. On Linux the function 'run_wrapper' runs process with stderr
connected to pipe, while on Windows stderr is mapped to stderr of the
running script.

When no output was made to stderr, this difference was not observable.
The new version of llvm-symbolizer (https://reviews.llvm.org/D149759)
complains about missing binary file, so stderr is not empty anymore and
the test fails on Windows and passes on Linux.

Author: spavloff

llvm/test/Support/interrupts.test

@@ -10,7 +10,8 @@ import sys
 import time
 
 def run_symbolizer():
-    proc = subprocess.Popen([sys.argv[2]], stdout=subprocess.PIPE, stdin=subprocess.PIPE)
+    proc = subprocess.Popen([sys.argv[2]], stdout=subprocess.PIPE,
+                            stdin=subprocess.PIPE, stderr=sys.stderr)
     # Write then read some output to ensure the process has started fully.
     proc.stdin.write(b'foo bar\n')
     proc.stdin.flush()
@@ -34,8 +35,8 @@ def run_wrapper():
                                 startupinfo=startupinfo,
                                 creationflags=subprocess.CREATE_NEW_CONSOLE)
     else:
-        proc = subprocess.Popen(args,
-                                stderr=subprocess.PIPE)
+        proc = subprocess.Popen(args, stderr=sys.stderr)
+        proc.wait()
 
 if sys.argv[1] == 'wrapper':
     run_wrapper()