Consolidate test util functions

[dereknola]

Proposed Changes

Makes translating between docker and E2E tests more seemless.

• Moved RunCommand into central testing utility package
• Exported docker test Kubeconfig for similar usage by RunCommand that E2E already had
• Consolidated docker testing import to docker across all docker tests
• Move away from legacy ranchertest/mycontainer to a modern nginx/busybox image

Types of Changes

Verification

CI still green

Testing

Its all testing

Linked Issues

User-Facing Change

Further Comments

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 21.74%. Comparing base (68749aa) to head (420227a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13281      +/-   ##
==========================================
- Coverage   21.75%   21.74%   -0.01%     
==========================================
  Files         186      186              
  Lines       15284    15284              
==========================================
- Hits         3325     3324       -1     
- Misses      11520    11521       +1     
  Partials      439      439              

Flag	Coverage Δ
unittests	21.74% <ø> (-0.01%)	⬇️

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