Defaulting run_tests to just opt.
diff --git a/tools/run_tests/run_tests.py b/tools/run_tests/run_tests.py
index aee19cd..a132ef4 100755
--- a/tools/run_tests/run_tests.py
+++ b/tools/run_tests/run_tests.py
@@ -240,7 +240,7 @@
     }
 
 
-_DEFAULT = ['dbg', 'opt']
+_DEFAULT = ['opt']
 _LANGUAGES = {
     'c++': CLanguage('cxx', 'c++'),
     'c': CLanguage('c', 'c'),