QDeclarative tests: Introduce base class for data tests.

In tests/auto/shared/util.* replace macros/find functions by
a base class QDeclarativeDataTest with accessors for the
data directory helper functions to create URLs from it.

The class relies on QFINDTESTDATA, which is the standard
way of locating test data. Using the class should reduce
the number of calls to QFileInfo.exists(), etc significantly.

In addition, provide utility functions for messages.

Reviewed-by: Michael Brasser <michael.brasser@nokia.com>

Change-Id: Id2beacb157922ee9412f9e45cf9695cec1f8379a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>