What does this PR do?

proxytest from elastic-agent repo was moved to elastic-agent-libs which also included a bug fix in [this PR]#358). But a follow up PR to remove this package from elastic-agent was missed.

In the meantime, some changes were made to proxytest package on elastic-agent repo in this PR.

To avoid anymore future divergence and consolidate our changes into one package - I am updating the changes here. After this, will immediately delete the proxytest package on elastic-agent repo by this pull request

Why is it important?

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

Related issues

• Required for Move beatreceiver proxy test from beats to elastic-agent repo elastic-agent#12468