refactor(tests): share one soulfind fixture across the e2e suites

The TUI suite had grown its own copy of the soulfind spawner the protocol
suite already carried, and its own copy of the CLI suite's in-process
client and settle helpers. All four suites now spawn, log in, settle and
skip through `tests/common`: the CLI suite's own server goes too, and
the fixture checks its child is still alive before calling the port
answered, so a lost port cannot pass another server off as ours.

.github/workflows/ci.yml

@@ -111,8 +111,7 @@ jobs:
         run: cargo test -p soulseek-rs --test cli_e2e -- --nocapture
 
       # The window over a real session: keys in, a screen out, a peer on the
-      # other end. The unit tests drive it against a stand-in session and the
-      # CLI suite never opens it, so this is the only place the two meet.
+      # other end. Nothing else opens the TUI against a server.
       - name: Run TUI e2e tests (required against soulfind)
         env:
           SOULSEEK_E2E_REQUIRED: "1"