Improve assert_samples_in test failure output

=== Before ===

>       assert all_in, print_samples(samples)
E       AssertionError: None
E       assert False

test_utils.py:37: AssertionError
------------------------------------------------- Captured stdout call -------------------------------------------------
Using seed 2353196025
[211106778865792]
{211106778865792, 211106242027648, 70369290510464, 211106778898560}
['h1', 'f2', 'g2', 'e3', 'h3', 'f4', 'g6', 'h6']: 476
['h1', 'f2', 'g2', 'h2', 'e3', 'h3', 'f4', 'g6', 'h6']: 17
['h1', 'f2', 'g2', 'h2', 'e3', 'h3', 'g6', 'h6']: 5
['h1', 'f2', 'g2', 'e3', 'h3', 'f4', 'g6']: 2

=== After ===

>       assert all_in, message("Unexpected bitboard in samples")
E       AssertionError: Unexpected bitboard in samples
E       assert False

test_utils.py:47: AssertionError
------------------------------------------------- Captured stdout call -------------------------------------------------
Using seed 3713866156
Expected bitboards:
      [f2 g2 h2 g3 d6 f7 g7 h7 b8 d8 h8]
Sample distribution:
  493 [f2 g2 h2 g3 d6 f7 g7 h7 b8 d8 h8]
    7 [f2 g2 h2 g3 d6 f7 g7 h7 b8 h8]

unitary/quantum_chess/test_utils.py

@@ -17,28 +17,38 @@
 import unitary.quantum_chess.bit_utils as u
 
 
-def print_samples(samples):
-    """For debugging only. Prints all the samples as lists of squares."""
+def bitboard_to_string(bitboard):
+    """Returns space-separated string of square names."""
+    return "[" + " ".join(u.bitboard_to_squares(bitboard)) + "]"
+
+
+def print_samples(samples, possibilities):
+    """For debugging only. Prints expected and actual lists of squares."""
+    print('Expected bitboards:')
+    for bitboard in possibilities:
+        print(" " * 5, bitboard_to_string(bitboard))
+    print('Sample distribution:')
     sample_dict = {}
     for sample in samples:
         if sample not in sample_dict:
             sample_dict[sample] = 0
         sample_dict[sample] += 1
     for key in sample_dict:
-        print(f"{u.bitboard_to_squares(key)}: {sample_dict[key]}")
+        print(f"{sample_dict[key]:5} {bitboard_to_string(key)}")
 
 
 def assert_samples_in(b, possibilities):
+    def message(message):
+        print_samples(samples, possibilities)
+        return message
     samples = b.sample(500)
     assert len(samples) == 500
     all_in = all(sample in possibilities for sample in samples)
-    print(possibilities)
-    print(set(samples))
-    assert all_in, print_samples(samples)
+    assert all_in, message("Unexpected bitboard in samples")
     # make sure each is represented at least once
     for p in possibilities:
         any_in = any(sample == p for sample in samples)
-        assert any_in, print_samples(samples)
+        assert any_in, message("Not all expected bitboards appeared")
 
 
 def assert_sample_distribution(b, probability_map, p_significant=1e-6):