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/requirements.txt

@@ -1,5 +1,7 @@
 aiohttp~=3.9.3
 pytest~=8.3.3
+pytest-xdist~=3.6
+filelock~=3.16
 numpy~=1.26.4
 openai~=2.14.0
 prometheus-client~=0.20.0