TUN-10391: Avoid blocking cloudflared due to logging

Pipes have a finite OS buffer (\~64KB Linux, \~4KB macOS, \~4KB Windows). Since nobody was reading stdout/stderr during the process lifetime, cloudflared would block once the buffer filled up. The post-terminate()/read() could only get whatever fit in the buffer, causing truncated logs.

There was also a race between terminate() and read(): the process might not have flushed its final output yet.

We're also deleting `test_default_only`. Since we changed `edge-ip-version` to auto, this test became redundant.