fix(test): run the server suite on a private tmux server

Several suites start real `agenthub-lane-<id>` tmux sessions and then kill
or reset them, with lane ids from their own throwaway database. On the
user's default tmux server those names collide with the dashboard's real
lane panes, so a test run — including the one the pre-commit hook runs on
every commit — killed whichever live session shared an id, the pane the
commit was made from included.

`npm run test:server` now goes through scripts/test-server.js, which drops
TMUX, points TMUX_TMPDIR at a fresh directory and kills that server after.
Also fixes the ERUNTIMEOUT test's tmux mock, which had the wrong (async,
cmd+args) signature and only passed by accident.

package.json

@@ -62,8 +62,8 @@
     "import-history": "node scripts/import-history.js",
     "reconcile-tokens": "node scripts/import-history.js --reconcile-tokens",
     "clear-data": "node scripts/clear-data.js",
-    "test": "node --test server/__tests__/*.test.js && cd client && npm test",
-    "test:server": "node --test server/__tests__/*.test.js",
+    "test": "node scripts/test-server.js && cd client && npm test",
+    "test:server": "node scripts/test-server.js",
     "test:client": "cd client && npm test",
     "format": "prettier --write .",
     "format:check": "prettier --check .",