Support random availble port selection in DeviceEmulator

[BrianJKoopman]

Description

This PR adds support for binding to a random available port when using the 'tcp' relay in the DeviceEmulator. This is now the default behavior. Tests that use this will need to grab the bound port via emulator.port to connect to.

Motivation and Context

This should fix the issue with flaky moxa tests seen in #633.

How Has This Been Tested?

Tested locally with the modifications to the tests here.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[jlashner]

Looks good, thanks brian!

[BrianJKoopman]

This doesn't actually solve the problem in #633, so I might hold off on merging.

[BrianJKoopman]

Some very similar changes were made in #802, enabling random port selection, while maintaining the existing defaults. Closing, with remaining changes submitted in #934.