Adding test for IndexBinaryFlat.reconstruct_n() (#3310)

Summary:
Pull Request resolved: #3310

**Context**
[Issue 2751](#2751) is already fixed as class wrappers has replacement definition of reconstruct_n in handle_IndexBinary.

**In this diff**,
Writing test test_reconstruct for binary index to validate fix for above issue.

Reviewed By: junjieqi

Differential Revision: D55168600

fbshipit-source-id: b62dc5fa89d65b843c52faa7456f046142e34421

tests/test_index_binary.py

@@ -143,6 +143,15 @@ def test_range_search(self):
         # nb tests is actually low...
         self.assertTrue(nt1 > 19 and nt2 > 19)
 
+    def test_reconstruct(self):
+        index = faiss.IndexBinaryFlat(64)
+        input_vector = np.random.randint(0, 255, size=(10, index.code_size)).astype("uint8")
+        index.add(input_vector)
+
+        reconstructed_vector = index.reconstruct_n(0, 4)
+        assert reconstructed_vector.shape == (4, index.code_size)
+        assert np.all(input_vector[:4] == reconstructed_vector)
+
 
 class TestBinaryIVF(unittest.TestCase):