tst_QSvgHandler: fix some mem-leaks in testCreateAnimateTransformNode()

The test deleted `node` at the end of the test function, but didn't
take into account that there are various QEXPECT_FAIL(..., Abort)
in-between, which thus leak the node.

Fix by holding the node in a unique_ptr.

This doesn't get rid of all leaks in the function, but those are the
leaks that the test function itself is responsible for. The rest will
be fixed in a follow-up.

Amends c9b94e6bdb2b0a031cdba35375572ecf9e94e455 (6.12).

Pick-to: 6.12
Change-Id: I6bd7ad313d7d521f6bfca64f6d54c485732433e0
Reviewed-by: Robert Löhning <robert.loehning@qt.io>