OZP-103: Write RPC API unit tests.

1. Adding functional tests for RPC api.
2. Modified WidgetSpecBase to allow multiple widgets to be opened for a test.
3. Improved the wait after widgets are launched to wait for the modal backdrop to be gone instead of waiting 2 seconds always.
Merge branch 'feature/ozp-61-rpc-api'
Merge branch 'master' into feature/ozp-150-create-dashboard