utils: tweak blackbox test assertion message

[benoit-pierre]

I noticed that after exporting support for blackbox tests the assertion message on failure changed from:

_____________________________ TestBlackboxReplays.test_carry_upper_spacing6 ______________________________
bb = <test.test_blackbox.TestBlackboxReplays object at 0x14cb56555f98>
>   new_f = lambda bb: (f(bb), replay(bb, name, test))
E   AssertionError: test_carry_upper_spacing6
E
E       "TEFT": "{<}test",
E       "W-G": "{^ing\nwith}",
E
E     > TEFT/W-G  ' TESTING\nWITH'
E       W-G       ' TESTING\nwithing\nwith'
E
E   assert ' TESTING\nwith' == ' TESTING\nWITH'
E        TESTING
E     - with
E     + WITH
test/test_blackbox.py:117: AssertionError
================================== 1 failed, 132 passed in 0.91 seconds ==================================

to:

________________________________ TestsBlackbox.test_carry_upper_spacing6 _________________________________
bb = <test.test_blackbox.TestsBlackbox object at 0x154dd5d1fa58>
>   new_f = lambda bb: (f(bb), replay(bb, name, test))
E   AssertionError: test_carry_upper_spacing6
E
E     "TEFT": "{<}test",
E     "W-G": "{^ing\nwith}",
E
E   > TEFT/W-G  ' TESTING\nWITH'
E     W-G       ' TESTING\nwithing\nwith'
plover_build_utils/testing.py:113: AssertionError
================================== 1 failed, 132 passed in 0.79 seconds ==================================

So tweak it to add back the resulting output:

________________________________ TestsBlackbox.test_carry_upper_spacing6 _________________________________
bb = <test.test_blackbox.TestsBlackbox object at 0x151e97c14a58>
>   new_f = lambda bb: (f(bb), replay(bb, name, test))
E   AssertionError: test_carry_upper_spacing6
E
E     "TEFT": "{<}test",
E     "W-G": "{^ing\nwith}",
E
E   > TEFT/W-G  ' TESTING\nWITH'
E     W-G       ' TESTING\nwithing\nwith'
E
E      ' TESTING\nwith'
E   != ' TESTING\nWITH'
plover_build_utils/testing.py:118: AssertionError
================================== 1 failed, 132 passed in 0.75 seconds ==================================