Force flush streams in tests for #1514

This avoids the problem where writes to stderr/stdout stop being
line-buffered when stderr/stdout is redirected to a file, which led
to different order of outputs between Linux and Windows in our tests.

projects/SelfTest/Baselines/console.std.approved.txt

@@ -1,5 +1,5 @@
-Nor would this
 This would not be caught previously
+Nor would this
 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 <exe-name> is a <version> host application.