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/core/testing.d.ts

@@ -103,9 +103,15 @@ export declare class InjectSetupWrapper {
 /** @deprecated */
 export declare var it: any;
 
+/** @deprecated */
+export declare function resetBaseTestProviders(): void;
+
 /** @experimental */
 export declare function resetTestEnvironment(): void;
 
+/** @deprecated */
+export declare function setBaseTestProviders(platformProviders: Array<Type | Provider | any[]>, applicationProviders: Array<Type | Provider | any[]>): void;
+
 /** @stable */
 export declare class TestComponentBuilder {
     protected _injector: Injector;