Speed up engine262 test

[Jack-Works]

Hi! We recent fix engine262's RegExp implementation and boost buildString in regExpUtils.js by a native implementation.

You can call boostTest262Harness(realm) after a user land buildString is defined.

This is how we use it:

    test.attrs.includes.unshift('assert.js', 'sta.js');
    if (test.attrs.flags.async) {
      test.attrs.includes.unshift('doneprintHandle.js');
    }
    // evaluates all harness file required by the teset
    for (const include of test.attrs.includes) {
      if (includeCache[include] === undefined) {
        const p = path.resolve(TEST262, `harness/${include}`);
        includeCache[include] = {
          source: fs.readFileSync(p, 'utf8'),
          specifier: p,
        };
      }
      const entry = includeCache[include];
      const completion = realm.evaluateScript(entry.source, { specifier: entry.specifier });
      if (completion instanceof AbruptCompletion) {
        return fails(test, inspect(completion));
      }
    }
    // boost it
    boostTest262Harness(realm);

By using the boost, all RegExp related test can save 2 minutes on build the large string.