Recognize when LTP test returns zero

The previous code was not taking in consideration the return code 0, so
when test text was not acquired correctly, the text process ended up to
recognize test failure.

libkirk/ltp.py

@@ -362,7 +362,9 @@ async def read_result(
                 elif not error:
                     failed = 1
 
-        if retcode in (2, -1):
+        if retcode == 0:
+            status = ResultStatus.PASS
+        elif retcode in (2, -1):
             status = ResultStatus.BROK
         elif retcode == 4:
             status = ResultStatus.WARN