Refactor Calendar tests

[igdmdimitrov]

Closes #6453

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code
• This PR includes API docs for newly added methods/properties
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes
• This PR includes behavioral changes and the feature specification has been updated with them

[nrobakova]

1. You can create a new method in the helper calss called for example VerifyDateSelected and to use it instead of:

expect(
                weekDays[3].nativeElement.classList.contains(
                    'igx-calendar__date--selected'
                )
            ).toBe(true);

2. You can use directly calendar method or call triggerEventHandler instead of calling UIInteractions.simulateKeyDownEvent
3. You can move the tests 'Should create proper calendar model' and 'Should receive correct values from utility functions' in different describe, because they do not need a fixture and do not need to call compileComponents