schema,tests: add support for new system usernames

[alexclewontin]

Supports canonical/snapd#12141

[codecov-commenter]

Codecov Report

Merging #3964 (b44a93d) into main (41c2c9f) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head b44a93d differs from pull request most recent head cfdd28d. Consider uploading reports for the commit cfdd28d to get more accurate results

@@           Coverage Diff           @@
##             main    #3964   +/-   ##
=======================================
  Coverage   91.05%   91.05%           
=======================================
  Files          56       56           
  Lines        4091     4081   -10     
=======================================
- Hits         3725     3716    -9     
+ Misses        366      365    -1     

Impacted Files	Coverage Δ
snapcraft/meta/snap_yaml.py	96.36% <0.00%> (-0.04%)	⬇️
snapcraft/snap_config.py	100.00% <0.00%> (ø)
snapcraft/store/onprem_client.py	100.00% <0.00%> (ø)
snapcraft/providers.py	97.64% <0.00%> (+1.09%)	⬆️

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[mr-cal]

Thanks!