Fix crash in timeout signal handler within testing/test_env.py

Noticed this from:
https://chromium-swarm.appspot.com/task?id=3e5bfa5ea894bf10&refresh=10&show_raw=1&wide_logs=true

Post stating it's win32 only:
https://github.com/python/mypy/issues/3872

Bug: 733612
Change-Id: I7bd8fc20e4793a225e093450ba902ae525af8ac9
Reviewed-on: https://chromium-review.googlesource.com/1117858
Reviewed-by: Ben Pastene <[email protected]>
Reviewed-by: Dirk Pranke <[email protected]>
Commit-Queue: agrieve <[email protected]>
Cr-Commit-Position: refs/heads/master@{#570999}
diff --git a/testing/test_env.py b/testing/test_env.py
index 1b70351..bd0a874 100755
--- a/testing/test_env.py
+++ b/testing/test_env.py
@@ -204,7 +204,8 @@
   assert all(isinstance(p, subprocess.Popen) for p in procs)
   def _sig_handler(sig, _):
     for p in procs:
-      if sig == signal.SIGBREAK:
+      # SIGBREAK is defined only for win32.
+      if sys.platform == 'win32' and sig == signal.SIGBREAK:
         p.send_signal(signal.CTRL_BREAK_EVENT)
       else:
         p.send_signal(sig)