Documentation: Add instructions on how to provide a test262 diff for LibJS changes

[ADKaster]

We often find that a test262 diff is important for libjs changes

• This procedure is 'lightly' documented in the test262 runner repo
• It is hard for new contributors to make meaningful changes to libjs without good docs
• The CI does this in a not-very-discoverable manner

Follow-up: Should we add a bot command to request a test262 diff?

[devgianlu]

Very simple script I've used to run test262:

#!/usr/bin/env bash

set -e

if [ ! -d libjs-test262 ]; then
  git clone git@github.com:LadybirdBrowser/libjs-test262.git --depth 1
fi
if [ ! -d libjs-data ]; then
  git clone git@github.com:LadybirdBrowser/libjs-data.git --depth 1
fi
if [ ! -d test262 ]; then
  git clone git@github.com:tc39/test262.git --depth 1
fi
if [ ! -d test262-parser-tests ]; then
  git clone git@github.com:tc39/test262-parser-tests --depth 1
fi

./Toolchain/BuildVcpkg.sh

python3 -m venv venv
source venv/bin/activate

pip install -r libjs-test262/requirements.txt

mkdir -p old-libjs-data
cp -R libjs-data/test262 old-libjs-data

CC=clang-19 CXX=clang++-19 cmake --preset Debug -B libjs-test262/Build \
    -DCMAKE_C_COMPILER=clang-19 \
    -DCMAKE_CXX_COMPILER=clang++-19 \
    -DWASM_SPEC_TEST_SKIP_FORMATTING=ON \
    -DINCLUDE_WASM_SPEC_TESTS=OFF \
    -DENABLE_GUI_TARGETS=OFF

ninja -C libjs-test262/Build test262-runner test-js

cd libjs-test262
python3 run_all_and_update_results.py \
            --serenity .. \
            --test262 ../test262 \
            --test262-parser-tests ../test262-parser-tests \
            --results-json ../libjs-data/test262/results.json \
            --per-file-output ../libjs-data/test262/per-file-master.json
cd ..

./libjs-test262/per_file_result_diff.py -o old-libjs-data/test262/per-file-master.json -n libjs-data/test262/per-file-master.json

On Fedora,wat2wasm was installed with dnf install wabt.