Improve negative test cases for JSON generation

[riedgar-ms]

Switch the remaining negative test cases to the improved pattern devised by @hudson-ai . This looks for particular failure bytes in the input, rather than depending on the vagaries of models.Mock() behaviour.

[riedgar-ms]

And please, please squash this on merge. The intermediate commits have at best zero value.

[hudson-ai]

Looks good, thanks @riedgar-ms! I think that this is a lot more intuitive than the negative tests using Mock :)

Do you think that adding an explicit grammar.match to the positive cases (in addition to checking successful generation by Mock) would add any comfort/value?

[riedgar-ms]

Probably wouldn't hurt @hudson-ai . Actually, that should likely be most of the testing, since you want the test as 'close to the code' as possible. Still having some models.Mock() test cases is useful, to check more of the stack.

[Harsha-Nori]

LGTM