Rename New-TestContainer to New-PesterContainer and fix Run.Container usage

[nohwnd]

PR Summary

Fixed passing container object via the Run.Container property on the configuration object by moving the functionality to the New-TestContainer function, and renaming that function to New-PesterContainer.

Fix #1737

PR Checklist

• PR has meaningful title
• Summary describes changes
• PR is ready to be merged
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• All tests pass
  • Run ./build.ps1 -Clean; ./test.ps1 -NoBuild. Use a new PowerShell process when C# code is changed.
• Tests are added/update (if required)
• Documentation is updated/added (if required)

[nohwnd]

@fflaten the "all tests pass" point in TODO seems unnecessary. I don't mind if people develop against our own CI builds. There is barely any traffic, and I rarely run all tests locally myself. Should we remove it?

[fflaten]

Wasn't sure about it myself, so added it just in case. Remove.