Improve diff output when tests fail (#5152)

Use difflib's unified_diff to produce a unified diff between the
expected and actual file. This has the benefit of including the files
compared in the output which makes it easier to do further inspection on
them.

Author: dagnir

test/migration-tool-tests/src/test/resources/run-test

@@ -1,11 +1,12 @@
 #!/usr/bin/env python
 
+from difflib import unified_diff
 import subprocess
 import filecmp
 import os
 import shutil, errno
 import argparse
-import difflib
+import sys
 
 RESOURCE_DIR = os.path.join(os.path.dirname(os.path.dirname(os.path.abspath(__file__))), "resources")
 BEFORE_DIR = os.path.join(RESOURCE_DIR, "before")
@@ -34,14 +35,13 @@ def run_test(version):
 
 def compare_directory(dcmp):
     if dcmp.diff_files or dcmp.left_only or dcmp.right_only:
+        print("Unexpected diffs found!")
         for diff_file in dcmp.diff_files:
-            file1 = open(dcmp.right + "/" + diff_file, 'r')
-            file2 = open(dcmp.left + "/" + diff_file, 'r')
-            diff = difflib.ndiff(file1.readlines(), file2.readlines())
-            changes = [l for l in diff if l.startswith('+ ') or l.startswith('- ')]
-            print("Diff found in file: " + diff_file)
-            for change in changes:
-                print(change)
+            a = dcmp.right + "/" + diff_file
+            a_contents = open(a, 'r').readlines()
+            b = dcmp.left + "/" + diff_file
+            b_contents = open(b, 'r').readlines()
+            sys.stdout.writelines(unified_diff(a_contents, b_contents, fromfile=a, tofile=b))
 
         return False
     for sub_dcmp in dcmp.subdirs.values():