Automatically set --runInBand when running benchmarks

Summary:
When `--benchmarks` is passed to `yarn fantom`, automatically append
`--runInBand` to the Jest arguments. This ensures benchmarks run
sequentially in the same process, avoiding parallel test execution that
would introduce noise in performance measurements.

Changelog: [Internal]

Reviewed By: lenaic

Differential Revision: D100796022

fbshipit-source-id: b78aa44d65ef8618eec6fb08680b5b455cf2470b

Author: rubennorte

scripts/fantom.sh

@@ -27,4 +27,8 @@ for arg in "$@"; do
   esac
 done
 
+if [[ -n "$FANTOM_RUN_BENCHMARKS" ]]; then
+  ARGS+=("--runInBand")
+fi
+
 yarn jest --config private/react-native-fantom/config/jest.config.js "${ARGS[@]}"