test: Add testBodyJSON, remove testBody.
The new function testBodyJSON ensures that the request body decodes as JSON
and that the resulting data structure is equal to a provided data structure.
This has several benefits over the previously used testBody function:
- The field order is ignored
- Any non-significant whitespace is ignored
- Valid JSON syntx is enforced
Since all uses of testBody were using it to check for JSON data, no call
sites remain and the function has been removed.