[assert_output/3] Highlight or isolate exactly which bit of the output doesn't match

[simonharris]

eg.

[plunit_assert] Output does not match expected: expected [[exactly,8,of,the,third,and,fourth,is,odd]], got [[exactly,1,of,the,third,and,fourth,is,odd]]

[simonharris]

And if possible, report the types found, because this is less than ideal:

ERROR: test parser:normalise_numbers: failed
[plunit_assert] Output does not match expected: expected [13], got [13]

Spoiler: one is a string and one an it