feat: consolidate headless-tests and browser-tests packages

[adklempner]

Description

Two separate packages are used in implementing the same "feature": browser simulation harness

User Story

As a developer I want to read and write code within a single package to work with browser simulations

Proposed Solution / Feature Design

Consolidate headless-tests and browser-tests into one package (their separation is an artifact of a previous browser testing architecture)

Optional: Diagram or Draft of Design

Notes