Add tests for QUnit.config.fixture

[Krinkle]

Documentation: https://qunitjs.com/api/config/fixture/

Source code:

qunit/src/html-runner/fixture.js

Lines 12 to 14 in 57c2dbc

	function storeFixture () {
	// Avoid overwriting user-defined values
	if (config.fixture !== undefined) {

Tests for HTML support reside in the test directory:
https://github.com/qunitjs/qunit/tree/57c2dbcffc694bf3a0b5d1d57e7f43f16ff29862/test

List any new HTML file in Gruntfile, and run via npx grunt test.

[Krinkle]

9cd5890