slabtop: add -o/--once

[cakebaker]

This PR adds -o/--once, though the functionality itself was already there. It's the same we currently use for when there is no argument specified.

[cakebaker]

Changes since last push:

• disabled tests using run_ucmd_as_root on Windows

[cakebaker]

Changes since last push:

• made the tests using run_ucmd_as_root linux-only

[cakebaker]

Changes since last push:

• fixed merge conflict

[cakebaker]

Changes since last push:

• made the *_as_non_root tests linux-only, too

[cakebaker]

The remaining clippy errors are fixed in #66 The clippy errors have been fixed in the meantime.

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 30 lines in your changes are missing coverage. Please review.

Project coverage is 46.77%. Comparing base (e2d7ed6) to head (2c68b9c).
Report is 11 commits behind head on main.

❗ Current head 2c68b9c differs from pull request most recent head bcfc147

Please upload reports for the commit bcfc147 to get more accurate results.

Files	Patch %	Lines
src/uu/slabtop/src/slabtop.rs	4.34%	21 Missing and 1 partial ⚠️
tests/by-util/test_slabtop.rs	70.37%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
- Coverage   51.30%   46.77%   -4.53%     
==========================================
  Files          18       18              
  Lines        2306     2313       +7     
  Branches      333      333              
==========================================
- Hits         1183     1082     -101     
+ Misses       1060     1051       -9     
- Partials       63      180     +117     

Flag	Coverage Δ
macos_latest	?
ubuntu_latest	47.43% <40.00%> (+0.86%)	⬆️
windows_latest	0.10% <0.00%> (-0.01%)	⬇️

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