Add test case for checking ConnectInBackground is set

Add test case for checking ConnectInBackground is set

ConnectInBackground should be set when opening a network session
due to a background request. This test checks that.
Unfortunately, none of the bearer plugins currently in Qt Base support
this attribute, so the test result is inconclusive.
(testing with a debugger shows the attribute is set correctly, but
it can't be read back as the set is discarded by the plugin
implementation)

Change-Id: Idcf777fe489a62d4ff5007ffd291a84ba052311b
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>