Fix path to AppRTC/collider on Windows

Bug: webrtc:7602
No-Try: True
Change-Id: I4d8f254e1316481f35638a1a2882275dfec2b5c1
Reviewed-on: https://webrtc-review.googlesource.com/66860
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22735}

rtc_tools/testing/utils.py

@@ -118,3 +118,7 @@ def GetPlatform():
   if sys.platform.startswith('darwin'):
     return 'mac'
   raise Exception("Can't run on platform %s." % sys.platform)
+
+
+def GetExecutableExtension():
+  return '.exe' if GetPlatform() == 'win' else ''