function defineCE(klass) {
  const tag = `test-${defineCECounter}`;
  customElements.define(tag, klass);
  defineCECounter += 1;
  return tag;
}
Registers a new element with an automatically generated unique name.
Helps to make a test fully isolated.

@example
const tag = defineCE(class extends MyMixin(HTMLElement) {
  // define custom element class body
});
const el = fixture(`<${tag}></${tag}>`);
// test el

@template {HTMLElement} T
@param {import("@open-wc/dedupe-mixin").Constructor<T>} klass Class which extends HTMLElement
@returns {string} Tag name of the registered element