This PR addresses an issue that several users have reported. If the
local oauth login server in one codex instance is left running (e.g. the
user abandons the oauth flow), a subsequent codex instance will receive
an error when attempting to log in because the localhost port is already
in use by the dangling web server from the first instance.
This PR adds a cancelation mechanism that the second instance can use to
abort the first login attempt and free up the port.
this dramatically improves time to run `cargo test -p codex-core` (~25x
speedup).
before:
```
cargo test -p codex-core 35.96s user 68.63s system 19% cpu 8:49.80 total
```
after:
```
cargo test -p codex-core 5.51s user 8.16s system 63% cpu 21.407 total
```
both tests measured "hot", i.e. on a 2nd run with no filesystem changes,
to exclude compile times.
approach inspired by [Delete Cargo Integration
Tests](https://matklad.github.io/2021/02/27/delete-cargo-integration-tests.html),
we move all test cases in tests/ into a single suite in order to have a
single binary, as there is significant overhead for each test binary
executed, and because test execution is only parallelized with a single
binary.
