Bump cmake version and fix some warnings

• remove some variables in endforeach, they're optional since cmake 3.14 and it was complaining about a mismatched one
• add the file extension to some ecm_add_tests to fix a lot of CMP0115 warnings
• bump the minimum supported cmake version to 3.16, as required by FindPoppler.cmake. 3.16 is available on the builders and in Ubuntu 20.04

Test Plan

Make sure nothing breaks on the supported platforms, I guess.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.