Update Schema Compare dialog to start a new connection (#15193)

* Update SC dialog to start a new connection

* Functionally complete

* Fix target db to pick correct database

* Address comments

* Added test+fixed one missing scenario

* Address comments + add one more test

extensions/schema-compare/src/test/testSchemaCompareDialog.ts

@@ -0,0 +1,34 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import * as azdata from 'azdata';
+import * as vscode from 'vscode';
+import { SchemaCompareDialog } from '../dialogs/schemaCompareDialog';
+import { SchemaCompareMainWindow } from '../schemaCompareMainWindow';
+
+export class SchemaCompareDialogTest extends SchemaCompareDialog {
+
+	constructor(
+		schemaCompareMainWindow: SchemaCompareMainWindow,
+		view: azdata.ModelView,
+		extensionContext: vscode.ExtensionContext) {
+		super(schemaCompareMainWindow, view, extensionContext);
+	}
+
+	// only for test
+	public getSourceServerDropdownValue(): string | azdata.CategoryValue {
+		if (this.sourceServerDropdown) {
+			return this.sourceServerDropdown.value;
+		}
+		return undefined;
+	}
+
+	public getTargetServerDropdownValue(): string | azdata.CategoryValue {
+		if (this.targetServerDropdown) {
+			return this.targetServerDropdown.value;
+		}
+		return undefined;
+	}
+}