Prefer detected test directory and relay URLs in integration tests

[chavic]

The integration tests use the store defaults, which are the public directories payjo.in and lets.payjo.in and public OHTTP relays run by third parties. #129 added a pull_request trigger, so every push to every PR now sends payjoin traffic through infrastructure other people operate.

Two costs:

1. Load on third-party services at a per-push cadence.
2. Our PR gate depends on their uptime. An outage there fails PRs for reasons unrelated to the change under review.

Raised in #129 (comment)

Related: #109 covers using the local directory and relay from test_tools. This is the same direction. The per-PR cadence from #129 makes it more pressing.