Override terminal size in doctests to standardise traceback format

Should fix test failures on Jenkins

IPython/testing/plugin/ipdoctest.py

@@ -28,6 +28,8 b' import sys'
 import traceback
 import unittest
 
+from testpath import modified_env
+
 from inspect import getmodule
 
 # We are overriding the default doctest runner, so we need to import a few
@@ -587,6 +589,8 b' class IPDocTestRunner(doctest.DocTestRunner,object):'
 
         test.globs.update(_ip.user_ns)
 
+        # Override terminal size to standardise traceback format
+        with modified_env({'COLUMNS': '80', 'LINES': '24'}):
         return super(IPDocTestRunner,self).run(test,
                                                compileflags,out,clear_globs)