Fix JSONX tests on 32bit

[JamesWrigley]

Also added a 32bit CI job.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.00%. Comparing base (52d8b90) to head (82562ab).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #403   +/-   ##
=======================================
  Coverage   90.00%   90.00%           
=======================================
  Files           7        7           
  Lines        1330     1330           
=======================================
  Hits         1197     1197           
  Misses        133      133           

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[quinnj]

Thanks!