Improve TestContext setup

Some RWT methods like getClient/getRequest/getResponse should not return
null if TestContext is used.

- set WebClient as selected client
- initialize ServiceContext with TestRequest and TestResponse

Change-Id: I6a3ccd0fb9349e0d1f7f55bcb19b37007782e27c
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>