fix(testing): reintroduce and deprecate setBaseTestProviders (#9905)

This change reverts the removal of setBaseTestProviders that was
introduced in 8d746e3f67.

Instead, setBaseTestProviders and the providers provided from
`@angular/platform-browser-dynamic/testing` and `@angular/server/testing`
will still work for the next release, but are deprecated.

See 8d746e3f67 for how to upgrade.

tools/public_api_guard/platform-browser/testing.d.ts

@@ -5,5 +5,8 @@ export declare class BrowserTestModule {
 /** @experimental */
 export declare const browserTestPlatform: () => PlatformRef;
 
+/** @deprecated */
+export declare const TEST_BROWSER_APPLICATION_PROVIDERS: Array<any>;
+
 /** @experimental */
 export declare const TEST_BROWSER_PLATFORM_PROVIDERS: Array<any>;