tests: enable pytest's builtin multiline diffing

...because it works on strings while doesn't work on byte arrays.

Change-Id: I2bb3b5a0a3d6ad965321c58fb90a02341db66d0f

tests/test_invocation.py

@@ -23,7 +23,7 @@ def test_example_invocation(capfdbinary, output, handler, args):
     expected = open(SRC_ROOT / 'tests' / 'invocation' / output, mode='rb').read()
     handler(args)
     captured = capfdbinary.readouterr()
-    assert captured.out == expected
+    assert captured.out.decode('utf-8') == expected.decode('utf-8')
     assert captured.err == b''