Mark line map exceptions as okay

This adds an entry to the default value for okay exceptions that
matches errors generated when we fail to map a file line comment.

This happens regularly in tests.  We log exceptions at debug level
(in case an interesting failure ever happens).  They are generally
safe to ignore.

Change-Id: I772e41efd56a01101db23f67754b69f6a9b88fbe

tests/base.py

@@ -2110,6 +2110,7 @@ class TestConfig:
             # We log git merge errors at debug level with tracebacks;
             # these are typically safe to ignore
             'ERROR: content conflict',
+            'mapLines',
         ]
         self.simple_layout = getattr(test, '__simple_layout__', None)
         self.gerrit_config = getattr(test, '__gerrit_config__', {})