server : change default pytest workers to 4 (#29376)

This commit changes the default number of pytest workers to 4 instead of
auto.

Refs: https://github.com/ggml-org/llama.cpp/pull/29369#issuecomment-5815050948

tools/server/tests/tests.sh

@@ -6,7 +6,7 @@ cd $SCRIPT_DIR
 
 set -eu
 
-WORKERS="${PYTEST_WORKERS:-auto}"
+WORKERS="${PYTEST_WORKERS:-4}"
 
 if [ $# -lt 1 ]
 then