[FEA] Enable full StringGen in test_structs_to_json

[andygrove]

Is your feature request related to a problem? Please describe.
In PR #9612, full StringGen is disabled due to a difference in encoding of two-digit unicode characters between cuDF and Spark.

Describe the solution you'd like
This issue is unlikely to impact users but means we do not have comprehensive testing because we can't use full StringGen. It would be nice if we could use StringGen and disable characters between 0x00 and 0xFF.

Describe alternatives you've considered

Additional context

[mattahrens]

Scope should be to fully support StringGen for all characters.

[mattahrens]

Added as low priority to this epic: #9.