opentelemetry-test-utils: add async base test class for asgi

[xrmx]

Description

This is required for trying to fix the issue below that is making the asgi tests work with latest release of asgiref.

Refs #2776

Type of change

Please delete options that are not relevant.

• 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)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test A

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[xrmx]

Another small batch of green tests is won using a small timeout instead of 0 in AsyncAsgiTestBase.get_output for self.communicator.receive_output(0.1). That is making the tests slower though.