Provide helper function for verifying suitable environment for running network autotests

Though apparently not documented anywhere, the network autotests are supposed to crash if an instance of the qt-test-server is not available. For symbian, at least, this doesn not happen and some tests take a very long time to terminate due to waiting for many connection attempts to timeout.

A much better approach would be for qtestlib to provide a shared function that each network-related test could include to verify that the test server is available. Such a function could be used to allow the tests to terminate gracefully and quickly, and to output a suitably descriptive error message and hints for resolving the problem.