run-tests: extract a `process_cmd_line` from the main function

The main function doing line comparison is quite complex. Slicing it in smaller
piece should clarify it.

(This is a gratuitous cleanup that I made while investigating a bug).

tests/run-tests.py

@@ -1695,6 +1695,18 b' class TTest(Test):'
                                 continue
                     postout.append(b'  ' + el)
 
+            pos, postout = self._process_cmd_line(cmd_line, pos, postout, after)
+
+        if pos in after:
+            postout += after.pop(pos)
+
+        if warnonly == WARN_YES:
+            exitcode = False # Set exitcode to warned.
+
+        return exitcode, postout
+
+    def _process_cmd_line(self, cmd_line, pos, postout, after):
+        """process a "command" part of a line from unified test output"""
             if cmd_line:
                 # Add on last return code.
                 ret = int(cmd_line.split()[1])
@@ -1704,14 +1716,7 b' class TTest(Test):'
                     # Merge in non-active test bits.
                     postout += after.pop(pos)
                 pos = int(cmd_line.split()[0])
-
-        if pos in after:
-            postout += after.pop(pos)
-
-        if warnonly == WARN_YES:
-            exitcode = False # Set exitcode to warned.
-
-        return exitcode, postout
+        return pos, postout
 
     @staticmethod
     def rematch(el, l):