ui: update tests to use new Angular Material test harnesses

[SuperITMan]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[X] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

Stark UI components are tested in an old way.

Expected behavior

Take benefit of new testing methods provided by Angular, with Angular Material test harnesses.

As mentioned in Angular Material CHANGELOG:

New @angular/cdk/testing infrastructure and Angular Material test harnesses

Testing components has historically relied on using implementation details such as CSS selectors to
find components and to trigger events. This meant that whenever a component library changed its
implementation, all of the tests relying on those components would need to be updated.

In version 9, we are introducing component harnesses, which offer an alternative way to test
components. By abstracting away the implementation details, you can make sure your unit tests are
correctly scoped and less brittle.

Most of Angular Material's components can now be tested via harnesses, and we are making harnesses
available to any component author as part of the Component Dev Kit (CDK).

Here's an example test before harnesses:

it('should switch to bug report template', async () => {
expect(fixture.debugElement.query('bug-report-form')).toBeNull();
    const selectTrigger = fixture.debugElement.query(By.css('.mat-select-trigger'));
    selectTrigger.triggerEventHandler('click', {});
    fixture.detectChanges();
    await fixture.whenStable();
    const options = document.querySelectorAll('.mat-select-panel mat-option');
    options[1].click(); // Click the second option, "Bug".
    fixture.detectChanges();
    await fixture.whenStable();
    expect(fixture.debugElement.query('bug-report-form')).not.toBeNull();
  });

And the same test with harnesses:

 it('should switch to bug report template', async () => {
    expect(fixture.debugElement.query('bug-report-form')).toBeNull();
    const select = await loader.getHarness(MatSelect);
    await select.clickOptions({text: 'Bug'});
    expect(fixture.debugElement.query('bug-report-form')).not.toBeNull();
  });

Learn more about
Angular Material's component harnesses
or building your own with the CDK.

What is the motivation / use case for changing the behavior?

Testing improvements

Environment

Angular version: > 9.x


Others: