extract test-utils as its own package for use with plugins, etc.

plugins/welcome/package.json

@@ -13,6 +13,7 @@
   "devDependencies": {
     "@backstage/cli": "^0.1.1-alpha.2",
     "@backstage/core": "^0.1.1-alpha.2",
+    "@backstage/theme": "^0.1.1-alpha.2",
     "@testing-library/jest-dom": "^4.2.4",
     "@testing-library/react": "^9.3.2",
     "@testing-library/user-event": "^7.1.2",
@@ -21,7 +22,8 @@
     "@types/testing-library__jest-dom": "5.0.2"
   },
   "peerDependencies": {
-    "@backstage/core": "^0.1.1-alpha.1",
+    "@backstage/core": "^0.1.1-alpha.2",
+    "@backstage/theme": "^0.1.1-alpha.2",
     "@material-ui/core": "^4.9.1",
     "@material-ui/icons": "^4.9.1",
     "@material-ui/lab": "4.0.0-alpha.45",

plugins/welcome/src/components/WelcomePage/WelcomePage.test.tsx

@@ -18,12 +18,8 @@ import React from 'react';
 import { render } from '@testing-library/react';
 import WelcomePage from './WelcomePage';
 import { ThemeProvider } from '@material-ui/core';
-import {
-  BackstageTheme,
-  ApiProvider,
-  ApiRegistry,
-  errorApiRef,
-} from '@backstage/core';
+import { BackstageTheme } from '@backstage/theme';
+import { ApiProvider, ApiRegistry, errorApiRef } from '@backstage/core';
 
 describe('WelcomePage', () => {
   it('should render', () => {