streamer: put testing_utilities.rs behind dev-context-only-utils

[kevinheavey]

Problem

As its name suggests, this module should be behind dev-context-only utils. This will also make it easier to remove the solana-sdk dependency

Summary of Changes

• Put testing_utils.rs behind dcou
• Update dependent crates accordingly

[KirillLykov]

Sounds good to me since it is used only in the scope of integration and unit tests

[KirillLykov]

@kevinheavey Regarding this dev-context-only, am I right that any test-related code should be under this feature in any crate in agave? I didn't notice when and why it was introduced. But lets say I want to add some code that, again, will be used for integration and unit tests. Shall it be under this feature?

[kevinheavey]

That is my understanding based on things @t-nelson has said