Enable cross-platform Send-MailMessage tests for CI

[ThreeFive-O]

PR Summary

Fixes #3950

This PR contains a cross-platform SMTP server mockup which allows to run the Send-MailMessage tests on the PowerShell CI system (Azure DevOps pipeline) as well as on local development machines for all supported platforms.

This PR includes:

• Use of netDumbster (see PR Context for more information) as a cross-platform SMTP server mockup
• Refactoring of Send-MailMessage tests with new Pester syntax
• Easier extensibility for further tests (Send-MailMessage cmdlets requires further test coverage)
• Added RequireSudoOnUnix tag as Linux and macOS on CI requires sudo for privileged port 25 (SMTP)
• Marked tests for named piping of objects into Send-MailMessage cmdlet as skipped, since this behaviour is not supported as of yet (see issue Send-MailMessage incorrectly handling ValueFromPipelineByPropertyName parameters #7591)

PR Context

Before this PR the Send-MailMessage tests run only on local Linux development machines when a local SMTP server is set up and running.

This is rather inconvenient for testing because:

• Test code is only meant to be run under Linux
• One has to set up its own SMTP server
• Tests are not supported on CI system (Azure DevOps pipeline) for PowerShell

The Azure DevOps pipeline VMs don't have any local SMTP server set up. Therefore the tests for Send-MailMessage are skipped during CI checks.

This PR will provide a way to run the tests on the Azure DevOps pipeline for Windows, Linux and macOS as well on local development machines.

PR Checklist

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• User-facing changes
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    • Issue filed:
• Testing - New and feature
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[iSazonov]

LGTM with one comment.

[iSazonov]

@adityapatwardhan Please look the PR.

[adityapatwardhan]

@ThreeFive-O The changes looks great, just added a comment for improving performance slightly. Everything else looks good.

[TravisEz13]

@adityapatwardhan Please review #9031 . I think we should still take this PR

[ThreeFive-O]

@adityapatwardhan and @TravisEz13 Not sure why my CLA status is still not reported, but all other checks have passed.

[TravisEz13]

@ThreeFive-O No worries, we got status on this commit: 0686809