Increase test coverage

[Cheukting]

The coverage now is not very good 😞 :

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So to make it better, do the following:

1. Pick a file that has < 80% coverage 👎🏻
2. Check the comment below to see if someone has already taken that file
3. Comment below which file you are going to work on. If you are only taking a part of the file, please state clearly which methods are yours.
4. After you have written new tests (that passes ✅ when run tox, details here), check the new coverage by running make coverage in your terminal (at the dir of the repo). The goal is 80% 💪🏻
5. When you send your PR, please link this issue by putting see #123 in the comment of your PR. Also, mention the new coverage. ⭐

Tips for writing the tests:

• Most of the functions have the JavaScript equivalent in the JS client, you can use the result of the JS client as the "answer" for the test
• You can use the TerminusDB console to check what the correct Json-LD of a query is (JS to Json-LD)

[Pradhvan]

Working on this issue 😄