Create tests for all formats supported by tablib

[marteinn]

Currently we have tests for:

• csv
• xls
• xlsx

But tablib also supports these formats (I've ranked them in perceived difficulty)

• json
• tsv
• dbf (I'm putting support on hold, until popular request changes my mind)
• latex (Supports only export: https://tablib.readthedocs.io/en/stable/formats/#latex)
• jira (Supports only export: https://tablib.readthedocs.io/en/stable/formats/#jira)
• rst (Supports only export: https://tablib.readthedocs.io/en/stable/formats/#rst)

So the task is, construct example file with a proper data source (some formats, such as rst/jira, requires more creative thinking) and put it in tests/files, then extend test case.

As you implement a format, tick the box.

[marteinn]

I've added test cases for .csv and .json. Might add test example for dbf in the future.