Investigate `headed` differences in the UI tests for Firefox

[jtpio]

In #6872 we noticed some differences for the rendering of the "Click to add a cell" button in Firefox when running the Playwright tests.

default (on CI)

The "Click to add a cell" button is displayed even when not actively hovering on it.

--headed

The "Click to add a cell" button is not displayed, which is the expected behavior.

We should consider investigating this more or report an issue on https://github.com/microsoft/playwright. Or check again later if the workaround can be removed in a future Playwright version:

notebook/ui-tests/test/utils.ts

Lines 38 to 48 in c77ebdc

	/**
	* Special case for firefox headless issue
	* See https://github.com/jupyter/notebook/pull/6872#issuecomment-1549594166 for more details
	*/
	export async function hideAddCellButton(
	page: IJupyterLabPageFixture
	): Promise<void> {
	await page
	.locator('.jp-Notebook-footer')
	.evaluate((element) => (element.style.display = 'none'));
	}