nesting level of some deeply nested tests decreased

[5saviahv]

.add function had some 3rd and 4th level nested tests.

• looking from output those indentations are rather subtle and no longer easily readable, I moved them to 2nd level and added argument descriptions used in tests.
• Also those deeply nested tests aren't very easy to follow in code.
• no actual change in tests, just formatting.

[fb55]

Nice PR! One comment seems to have been forgotten, otherwise this should be good to go.

[fb55]

Thanks for following up!