test: Enable assertions about prompt text shown to users

[SarahFrench]

Context

The testInputMap allows engineers to define a list of responses to be returned to prompts with matching Ids. When used in the context of a test this Input method would return values early based on the contents of testInputResponseMap.

This PR

This PR is part of breaking down #38205. In that PR we're implementing a security feature that prompts users to review downloaded providers. This includes presenting the user with hash data, so testing that feature should include making some assertions about the prompts users see.

In this PR I update the testInputMap helper to return a buffer containing the prompt text that a user would see, which can optionally be used for assertions in tests. To enable filling that buffer with the prompt text I have moved the point in func (i *UIInput) Input where testInputResponseMap is used to below where the prompt is written to the input io.Writer.

I've updated a test showing how this can be used.

Also, I've updated the testInputMap helper to queue cleanup internally, so calling code doesn't need to handle the returned close function.

Target Release

N/A

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.