test: start_time to be defined before clean() and setup()

Just in case any of these fails it has to be defined.

Signed-off-by: Jan Michalski <[email protected]>

src/test/unittest/basetest.py

@@ -168,6 +168,7 @@ def _execute(self, c):
             c (Context): test context
         """
         self.ctx = c
+        start_time = None
 
         try:
             # pre-execution cleanup