[Test] Fix for JSON tests

[riedgar-ms]

We have been getting intermittent failures in the JSON tests. These are occurring in the 'number' and 'integer' tests, and are almost certainly due to the behaviour of the model.Mock class.

Once its supplied string has been exhausted, Mock generates random characters (or to be more precise, produces randomised logits for the valid tokens). This means that every now and then, a digit can be produced, which will pass the grammar check for a number, but then fail our round-trip check. This is not a problem for any of our other tests, since in those cases, the final character is forced (e.g. " to close a string or ] to end an array), and there is no possible valid character afterwards.

The fix is to append a character to the string supplied to Mock which never appears in the one we supply from the test. This acts as a universal 'stop code.'

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.85%. Comparing base (bbabbca) to head (0d569a9).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #797      +/-   ##
==========================================
+ Coverage   55.34%   62.85%   +7.50%     
==========================================
  Files          55       55              
  Lines        4076     4076              
==========================================
+ Hits         2256     2562     +306     
+ Misses       1820     1514     -306     

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