Testing class that transforms JSON into a spreadsheet
Ideally,
the source data and the expected result of the function under test are both visible inside the test case.
That readers can see clearly what is being tested and what the function does.

When the source and result data are relatively big,
it's better to not include in the test case, 
but to keep in files in test resources.
This is usually not great, as readers lose visibility of what's really being tested,
but if large data pollutes the test case making it unreadable,
that's a worse option.

In your case, only the source data (json) is in the test case,
the result (xlsx) is outside.
In this case,
since the JSON is pretty unreadable in the code,
I recommend to move it to a resource file, like you did with the xlsx.
I think that's the most maintainable option for you.