Add missing integration tests for the Checkbox label

[vursen]

At the present time, the Checkbox's integration tests cover only the case when the label is set right after the checkbox's creation within a single round-trip:

The setLabelAsHtml method:

flow-components/vaadin-checkbox-flow-parent/vaadin-checkbox-flow-integration-tests/src/main/java/com/vaadin/flow/component/checkbox/tests/CheckboxDemoPage.java

Lines 106 to 115 in 65ccf0b

	private void addCheckboxWithHtmlLabel() {
	// begin-source-example
	// source-example-heading: Checkbox with simple html markup in the label
	Checkbox checkbox = new Checkbox();
	checkbox.setLabelAsHtml(
	"Accept the <a href='https://vaadin.com/privacy-policy'>privacy policy</a>");
	// end-source-example
	addCard("Checkbox with simple html markup in the label", checkbox);
	checkbox.setId("html-checkbox");
	}

The setLabel method:

flow-components/vaadin-checkbox-flow-parent/vaadin-checkbox-flow-integration-tests/src/main/java/com/vaadin/flow/component/checkbox/tests/CheckboxDemoPage.java

Lines 37 to 45 in 65ccf0b

	private void addDefaultCheckbox() {
	// begin-source-example
	// source-example-heading: Default Checkbox
	Checkbox checkbox = new Checkbox();
	checkbox.setLabel("Default Checkbox");
	// end-source-example
	addCard("Default Checkbox", checkbox);
	checkbox.setId("default-checkbox");
	}

However, there is another case when the user may want to change the label on an existing checkbox (that has already been sent to the client once).

This case is unfortunately not covered now that might result in undesirable passing Flow integrations tests with the new accessible <vaadin-checkbox> (see vaadin/web-components#2539). They shouldn't pass because the way the Flow Checkbox sets labels now (by overriding textContent or innerHTML) unintentionally removes the slotted <label> element that should end up throwing an exception.