server : use pytest-xdist for server tests (#28298)

* server : use pytest-xdist for server tests

This commit adds pytest-xdist to the server tests. This is pytest
plugin that distributes test execution across multiple CPU cores.

Assisted-by: pi:llama.cpp/qwen3.8-27B

Refs: https://github.com/ggml-org/llama.cpp/pull/26734#issuecomment-5220707042

* remove server_base_port and BASE_PORT

* use worksteal and pytest builting tmp_path

tools/server/tests/utils.py

@@ -294,6 +294,7 @@ class ServerProcess:
             server_args.append("--backend_sampling")
         if self.gcp_compat:
             env["AIP_MODE"] = "PREDICTION"
+            env["AIP_HTTP_PORT"] = str(self.server_port)
 
         args = [str(arg) for arg in [server_path, *server_args]]
         print(f"tests: starting server with: {' '.join(args)}")