While preparing problems with subtasks using Polygon, it's super annoying that we cannot set test groups using test scripts.

As a workaround, I wrote a simple script.

Test group of each test = the last token of the test generation command

function setPolygonTestGroupAutomatically() {
  const mapping = {};
  $("#testsTable tbody tr pre")
    .each((idx_minus_one, v) => {
      if (v.id) return;                             // Ignores raw input, only consider tests generated with command
      const tokens = v.innerText.split(" ");        // Test generation command
      const testGroup = tokens[tokens.length - 1];  // Last token of the test generation command
      if (!mapping[testGroup]) {
        mapping[testGroup] = [];
      }
      mapping[testGroup].push(idx_minus_one + 1);
    });
  const keyToIndices = Object.entries(mapping);
  function go (idx) {
    if (idx == keyToIndices.length) {
      console.log("Changed everything!"); 
      return;
    }
    const [groupName, indices] = keyToIndices[idx];
    $.post("/data/tests", {
      action: "setMultipleTestGroup",
      session: $('#session').text(),
      testset: Object.fromEntries(new URLSearchParams($("#add-test").attr('href')))['testset'],
      requestString: indices.map(idx => `testIndex=${idx}`).join('&'),
      groupName,
      ccid: $('meta[name="ccid"]').attr('content')
    }).error((err) => console.error(err))
    .then((data) => {
      console.log(`Done`, groupName, indices, data);
      setTimeout(go, 1000, idx+1);
    });
  }
  go(0);
}
setPolygonTestGroupAutomatically();

Paste the script above to the developer console on the tests page.

UPD: Added 1-second delay between each API call. It seems if we call the API multiple times simultaneously, Polygon will automatically change the dollars of the test script into non-consecutive numbers, making the whole working copy inaccessible.