Fix Linter: Test.output_dir

`Test.output_dir`:
This member was never properly defined and just gets hooked in
during builds. Define to an empty string to silence and make aware.

Author: ax3l

suite.py

@@ -42,6 +42,7 @@ def __init__(self, name):
         self.log = None
 
         self.buildDir = ""
+        self.output_dir = ""
 
         self.extra_build_dir = ""