test: keep pytest basetemp within the AF_UNIX path limit

Moving TMPDIR into the private runtime root left pytest's default
<TMPDIR>/pytest-of-<user>/pytest-<n> beneath it. With xdist's popen-gw<n>
the real-tmux witness bound a 110-byte socket path, over Linux's 107-byte
sun_path limit, so it failed under every worker count while passing
serially.

The controller now roots basetemp at the private root's pytest directory;
xdist hands workers popen-gw<n> beneath it. An explicit --basetemp wins.
A tmux-independent witness binds a socket at the same path budget.

tests/conftest.py

@@ -351,7 +351,8 @@ def context_probe_ledger(_no_context_window_network_probe):
     return _no_context_window_network_probe
 
 
-@pytest.hookimpl(specname="pytest_configure")
+# Before pytest's tmpdir plugin reads the basetemp this sets.
+@pytest.hookimpl(specname="pytest_configure", tryfirst=True)
 def pytest_configure_worker_runtime(config):
     configure_runtime(config, _runtime_environment)