test(tabset): test oriantation default value (#3520)

This minor change make sure test covers all default properties of the
tabset configuration.

src/tabset/tabset-config.spec.ts

@@ -6,5 +6,6 @@ describe('ngb-tabset-config', () => {
 
     expect(config.type).toBe('tabs');
     expect(config.justify).toBe('start');
+    expect(config.orientation).toBe('horizontal');
   });
 });