testharness.js throws in some cases

[asamuzaK]

When I tried to update wpt for jsdom, I noticed that testharness.js throws an exception in some tests.

TypeError: Cannot read properties of undefined (reading 'arguments.length')
    at do_substitution (http://web-platform.test:10000/resources/testharness.js:4417:49)
    at substitute_children (http://web-platform.test:10000/resources/testharness.js:4448:32)
    at substitute_single (http://web-platform.test:10000/resources/testharness.js:4461:13)
    at substitute (http://web-platform.test:10000/resources/testharness.js:4399:20)
    at substitute_children (http://web-platform.test:10000/resources/testharness.js:4439:39)
    at substitute_single (http://web-platform.test:10000/resources/testharness.js:4461:13)
    at substitute (http://web-platform.test:10000/resources/testharness.js:4399:20)
    at substitute_children (http://web-platform.test:10000/resources/testharness.js:4439:39)
    at substitute_single (http://web-platform.test:10000/resources/testharness.js:4461:13)
    at substitute (http://web-platform.test:10000/resources/testharness.js:4399:20)

On line 4417, it tries to extract an element from the substitutions[] array, but there are cases where substitutions is not an array but undefined, so the error occurred there.

This can be fixed by changing line 4407 to set a default value.

function substitute_single(template, substitutions = [])
{
  // snip
}