Ensure testcase filtering on the API side returns the three duration fields

[mfonism]

This is a more readable/simplified version of PR #2530.

Refs #1923

[mfonism]

I can squash the two commits into one if this looks good.

If we need to ensure filtering on the duration fields works on the API side, I can remove the second commit and leave just d18483b.

[mfonism]

@atodorov I've implemented the changes you requested.

I Casted the duration field values to float (instead of str) because I realized the database converts them to milliseconds first before casting them into the requested type. And I thought it would be more appropriate to get the milliseconds as floats instead of strings.

[mfonism]

@atodorov the failure on postgres comes from its inability to cast the duration fields to integer fields or float fields.

https://github.com/kiwitcms/Kiwi/pull/2531/checks?check_run_id=3667534662#step:5:1348

[mfonism]

@atodorov please see the changes.

[atodorov]

Functionality wise looks good but see the failing black, flake8 and pylint checks. They are all related.