[polymetis] Simplified integration tests

## Type of Issue

Select the type of issue:
- [ ] Bug report (to report a bug)
- [ ] Feature request (to request an additional feature)
- [ ] Tracker (I am just using this as a tracker)
- [x] Refactor request
- [ ] Documentation Ask

## Description

In #926, I split integration tests into 3 shell scripts. Currently, it's a bit awkward because process lifecycle (server, robot client, user client) is being managed by shell scripts; after fbrp integration, we should revisit this.

[Some ideas](https://github.com/facebookresearch/fairo/pull/926#discussion_r777861745) for removing tech debt.