Modify test parity check to be runnable on Mac

- Script used `mapfile` command, which is unavailable on Mac bash
- Replace with `while` loop intended to fulfill the same role

Assisted-by: Codex

Author: datalogics-cgreen

scripts/check_test_parity.sh

@@ -15,7 +15,12 @@ if ! git rev-parse --verify "$head_ref" > /dev/null 2>&1; then
     exit 1
 fi
 
-mapfile -t test_files < <(
+test_files=()
+while IFS= read -r file; do
+    if [[ -n "$file" ]]; then
+        test_files+=("$file")
+    fi
+done < <(
     git diff --name-only "$base_ref..$head_ref" -- tests | grep -E '\.py$' || true
 )