Fix bpo-19217: Calling assertEquals for moderately long list takes too long

[srinivasreddy]

https://bugs.python.org/issue19217

[matrixise]

@srinivasreddy The title of your PR is wrong, why "Closes" ?

[eamanu]

Its ok for me

[lisroach]

Is this still adding the header mentioned in the issue:

---

+++
@@ -1 +1 @@

? I do find that to look odd. Could this be improved?

[erlend-aasland]

@lisroach Yes, this is still adding the diff header. Could be improved by adding fromfile='expected', tofile='actual' to difflib.unified_diff() in Lib/unittest/case.py and Lib/unittest/test/test_case.py.

Example output:

--- expected
+++ actual
@@ -9997,4 +9997,4 @@
  1,
  1,
  1,
- 1]
+ 2]