Builds the ?filter=… query that carries the test filter (-t/--filter/-m/--module) into the page.

This is the only channel that works: QUnit evaluates config.filter at test declaration time, and its html-reporter block unconditionally overwrites config.filter from location.search at bundle-eval time — so a preconfig global would be clobbered. Returns '' when no filter is set, leaving URLs byte-identical to before.

import * as RunState from '../setup/run-state.ts';

const state = RunState.create();
buildQUnitFilterQuery({ filter: 'cart checkout', state }); // '?filter=cart+checkout'
buildQUnitFilterQuery({ state }); // '' — URL byte-identical to an unfiltered run