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/unit/test_tool_call.py

@@ -21,7 +21,6 @@ def create_server():
     global server
     server = ServerPreset.tinyllama2()
     server.model_alias = "tinyllama-2-tool-call"
-    server.server_port = 8081
     server.n_slots = 1
     server.n_ctx = 8192
     server.n_batch = 2048