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/tests.sh

@@ -6,13 +6,15 @@ cd $SCRIPT_DIR
 
 set -eu
 
+WORKERS="${PYTEST_WORKERS:-auto}"
+
 if [ $# -lt 1 ]
 then
     if [[ "${SLOW_TESTS:-0}" == 1 ]]; then
-        pytest --durations=30 -v -x
+        pytest --durations=30 -v -x -n "${WORKERS}" --dist=worksteal
     else
-        pytest --durations=30 -v -x -m "not slow"
+        pytest --durations=30 -v -x -n "${WORKERS}" --dist=worksteal -m "not slow"
     fi
 else
-    pytest --durations=30 "$@"
+    pytest --durations=30 -n "${WORKERS}" --dist=worksteal "$@"
 fi