[webapi] Add back 2 tests for WebRTC and remove duplicated ones

- RTCPeerConnection_iceGatheringState.html is covered by RTCPeerConnection_basic.html
- RTCPeerConnection_negotiationneeded_event.html duplicates to blink/RTCPeerConnection-onnegotiationneeded.html
- The setIdentityProvider and getIdentityAssertion attributes are not supportted

Impacted tests: New 2, Update 0, Delete 2
Unit test platform: IVI
Unit test result summary: Pass 0, Fail 2, Blocked 0