Added tests for newer_pairwise_group().

Cherry-picked from pypa/setuptools@a40114a.

distutils/tests/test_modified.py

@@ -1,7 +1,7 @@
 """Tests for distutils._modified."""
 import os
 
-from distutils._modified import newer, newer_pairwise, newer_group
+from distutils._modified import newer, newer_pairwise, newer_group, newer_pairwise_group
 from distutils.errors import DistutilsFileError
 from distutils.tests import support
 import pytest
@@ -89,3 +89,30 @@ def test_newer_group(self):
         assert not newer_group([one, two, old_file], three, missing='ignore')
 
         assert newer_group([one, two, old_file], three, missing='newer')
+
+
+@pytest.fixture
+def groups_target(tmpdir):
+    """Sets up some older sources, a target and newer sources.
+    Returns a 3-tuple in this order.
+    """
+    creation_order = ['older.c', 'older.h', 'target.o', 'newer.c', 'newer.h']
+    mtime = 0
+
+    for i in range(len(creation_order)):
+        creation_order[i] = os.path.join(str(tmpdir), creation_order[i])
+        with open(creation_order[i], 'w'):
+            pass
+
+        # make sure modification times are sequential
+        os.utime(creation_order[i], (mtime, mtime))
+        mtime += 1
+
+    return creation_order[:2], creation_order[2], creation_order[3:]
+
+
+def test_newer_pairwise_group(groups_target):
+    older = newer_pairwise_group([groups_target[0]], [groups_target[1]])
+    newer = newer_pairwise_group([groups_target[2]], [groups_target[1]])
+    assert older == ([], [])
+    assert newer == ([groups_target[2]], [groups_target[1]])