Add toxiproxy and adjust test helpers

[grcooper]

New:

• Added toxiproxy gem
• Added toxiproxy startup script for ci tests

Replace/Fixes:

• Replaced some memcached_mock tests with Toxiproxy tests
• Renamed some test helper params to be named params instead of positional arguments for easier use

Toxiproxy: https://github.com/Shopify/toxiproxy

• This is a gem that allows you to add jitter, response delays, make servers go down etc.

[juice928]

👋 Hi, I'm an automated AI code review bot. I ran some checks on this PR and found 1 point that might be worth attention (could be false positives, please use your judgment):

1. The script currently downloads an external binary without a verification step
   • bin/start-toxiproxy:L12: bin/start-toxiproxy:L12
   • Impact: Downloading and executing files directly could potentially introduce security risks to the supply chain if the upstream source is ever compromised.
   • Suggestion: Consider adding a step to verify the SHA256 checksum of the downloaded toxiproxy-server binary against a known, trusted hash before execution.

If you find these suggestions disruptive, you can reply "stop" , and I'll automatically skip this repository in the future.