Replace stop() and start() with assert.async() pattern
Replace stop() and start() with assert.async() pattern

Also update sl-tab-panel test code to reflect more recent testing
pattern standards.

azizpunjani/sl-ember-components,theoshu/sl-ember-components,softlayer/sl-ember-components,notmessenger/sl-ember-components,softlayer/sl-ember-components,juwara0/sl-ember-components,alxyuu/sl-ember-components,juwara0/sl-ember-components,alxyuu/sl-ember-components,SpikedKira/sl-ember-components,Suven/sl-ember-components,Suven/sl-ember-components,SpikedKira/sl-ember-components,theoshu/sl-ember-components,notmessenger/sl-ember-components,erangeles/sl-ember-components,azizpunjani/sl-ember-components,erangeles/sl-ember-components