run-test: enable checker in dev mode

Also, use explicit flush (instead of passing flush parameter to print)
to enable better compatibility across python versions.

Change-Id: Iaf294d88e932b778d5dce7f3c2b8eca775849973
diff --git a/tools/checker.py b/tools/checker.py
index 55f015e..5744c15 100755
--- a/tools/checker.py
+++ b/tools/checker.py
@@ -110,9 +110,10 @@
       text = Logger.Color.terminalCode(color, out) + text + \
              Logger.Color.terminalCode(Logger.Color.Default, out)
       if newLine:
-        print(text, flush=True, file=out)
+        print(text, file=out)
       else:
-        print(text, end="", flush=True, file=out)
+        print(text, end="", file=out)
+      out.flush()
 
   @staticmethod
   def fail(msg, file=None, line=-1):