surrogate pair tests: use escape sequences

interestingly, node's buffer doesn't throw when attempting to encode these half surrogates, it just generates utf8 garbage...

the reason the (testling) tests weren't passing is related to this: when browserifying, the test source actually has to pass through node's buffers and so the tests themselves were being improperly generated.

this patch fixes that by using unicode escape sequences in the tests instead of the actual characters.