chore: move overlay-container specs (#7080)

It seems like the overlay container tests got left behind during the move to the CDK. These changes move them next to the container.

Author: crisbeto

src/lib/core/overlay/overlay-container.spec.ts renamed to src/cdk/overlay/overlay-container.spec.ts

@@ -1,6 +1,6 @@
 import {async, inject, TestBed} from '@angular/core/testing';
 import {Component, NgModule, ViewChild, ViewContainerRef} from '@angular/core';
-import {PortalModule, TemplatePortalDirective} from '../portal/portal-directives';
+import {PortalModule, TemplatePortalDirective} from '@angular/cdk/portal';
 import {Overlay, OverlayContainer, OverlayModule} from './index';
 
 describe('OverlayContainer', () => {