Simple API Test, bump php dependencies

[jotoeri]

Initially i wrote this test in #1126, now splitting again for easier review.

• Adding simple API-Querying "Tests". They are by far not complete (in the sense of testing every possibility) and unfortunately not respected by the coverage, but they at least query each API-Request in full depth once, so better than nothing...^^
• In combination with these tests -> dropping php7.2 to use guzzle, thus also enabling to update to phpunit 9

[jotoeri]

Waiting for the big rebase once #1159 is merged.

[skjnldsv]

/rebase

[skjnldsv]

I created standards tests for phpunit in our github repo if you want 😉
https://github.com/nextcloud/forms/actions/new?category=owner

[jotoeri]

!! This one is based on #1087 and #1126, so the last one to merge then... 😉 !!

• Also reinclude composer.lock again (need my other pc 😉 )

[codecov]

Codecov Report

Merging #1148 (6ee2884) into fix/apiv2 (250b414) will decrease coverage by 0.65%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##             fix/apiv2    #1148      +/-   ##
===============================================
- Coverage        32.29%   31.64%   -0.66%     
  Complexity         415      415              
===============================================
  Files               38       38              
  Lines             1604     1637      +33     
===============================================
  Hits               518      518              
- Misses            1086     1119      +33