Cleanup application context in TestContext#tearDown

Application context is activated and added to servlet context in
TestContext#setup. We should clean it up in tearDown similar to
Fixture#disposeOfApplicationContext.

Change-Id: I0272a12ccc40f5fbfce9e1ddb1096d1a1b05172e
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>