Move viewModel utilities into testNotebookEditor.ts

[Yoyokrazy]

we could consider moving these utilities into testNotebookEditor

Originally posted by @rebornix in #211861 (comment)

vscode/src/vs/workbench/contrib/notebook/test/browser/contrib/notebookOutlineViewProviders.test.ts

Lines 57 to 100 in aed4230

	function createCodeCellViewModel(version: number = 1, source = '# code', textmodelId = 'textId') {
	return {
	textBuffer: {
	getLineCount() { return 0; }
	},
	getText() {
	return source;
	},
	model: {
	textModel: {
	id: textmodelId,
	getVersionId() { return version; }
	}
	},
	resolveTextModel() {
	return this.model.textModel as unknown;
	},
	cellKind: 2
	} as ICellViewModel;
	}
	function createMarkupCellViewModel(version: number = 1, source = 'markup', textmodelId = 'textId', alternativeId = 1) {
	return {
	textBuffer: {
	getLineCount() { return 0; }
	},
	getText() {
	return source;
	},
	getAlternativeId() {
	return alternativeId;
	},
	model: {
	textModel: {
	id: textmodelId,
	getVersionId() { return version; }
	}
	},
	resolveTextModel() {
	return this.model.textModel as unknown;
	},
	cellKind: 1
	} as ICellViewModel;
	}

[Yoyokrazy]

additionally, change source parameter to take in string[]. leaves us with an accurate way to call getLineCount

vscode/src/vs/workbench/contrib/notebook/test/browser/contrib/notebookOutlineViewProviders.test.ts

Lines 57 to 60 in aed4230

	function createCodeCellViewModel(version: number = 1, source = '# code', textmodelId = 'textId') {
	return {
	textBuffer: {
	getLineCount() { return 0; }