test(date-range-picker): improve coverage and refactor #5732 (#7647)

[Lipata]

Closes #7477

Authored-by: Plamena Miteva & Boris Penkov

Some additional enhancements to the IgxDateRangePickerComponent are included in this PR:

• The picker will now accumulate all errors and return them as one object during validation.

• Also, in single input mode it will use only displayFormat for formatting the input's dates.

• Additionally, the picker's displayValue as well as its default placeholder (in single input mode) are governed by Angular's getLocaleDateFormat and formatDate. These two methods can fail if a user has defined a locale setting, but has not registered it in their application via the registerLocaleData function. In this case the picker will now log warnings and will revert to the browser's locale settings while still adhering to the defined displayFormat. If a user has not provided any displayFormat, the picker has a default, constant format which will be used instead.

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 (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• 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 (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them