ToolbarLineBreakView should not be rendered when grouping is enabled.

[maxbarnas]

Suggested merge commit message (convention)

Fix (ui): A '-' divider will not be rendered when grouping is enabled. Closes #8582.

[Reinmar]

Works fine, but you need to stub console.warn() so it does not bleed to the console:

.............................................
WARN: 'Detected `console.warn()`:', 'toolbarview-line-break-ignored-when-grouping-items', ['foo', '-', 'bar'], '
Read more: https://ckeditor.com/docs/ckeditor5/latest/framework/guides/support/error-codes.html#error-toolbarview-line-break-ignored-when-grouping-items'
Chrome 87.0.4280.88 (Linux x86_64) ToolbarView fillFromConfig() does not render line separator when the button grouping option is enabled FAILED
	Error: Detected `console.warn()`:
	    at console.<computed> [as warn] (webpack:///./build/.automated-tests/entry-point.js?:67:11)
	    at logWarning (webpack:///./packages/ckeditor5-utils/src/ckeditorerror.js?:153:10)
	    at eval (webpack:///./packages/ckeditor5-ui/src/toolbar/toolbarview.js?:21:3703)
	    at Array.map (<anonymous>)
	    at ToolbarView.fillFromConfig (webpack:///./packages/ckeditor5-ui/src/toolbar/toolbarview.js?:21:3214)
	    at Context.eval (webpack:///./packages/ckeditor5-ui/tests/toolbar/toolbarview.js?:502:9)