appveyor: show test output in the build log in realtime

.appveyor.yml

@@ -22,6 +22,10 @@ environment:
   #   -mx=1 : Fast compression.
   APPVEYOR_CACHE_ENTRY_ZIP_ARGS: -t7z -snl -mtc -mx=1
 
+  # Disable python stdout buffering, so test output shows up in the build log in
+  # realtime, rather than all at once when the test runner exits.
+  PYTHONUNBUFFERED: 1
+
   # Define some PowerShell helper functions which are used in the scripts below.
   # They're defined in an environment variable to reduce noise in the build log.
   PS_UTILS: |-