CMake: add TEST_PLUGIN to test plugins

To silence the warning:

CMake Warning at cmake/QtPluginHelpers.cmake:387 (message):
  The installable test plugin QGeoSatelliteInfoSourceTestPlugin is built
as
  part of a test suite, but is not marked as TEST_PLUGIN using the
respective
  argument.

  This warning will soon become an error.
Call Stack (most recent call first):
  tests/auto/satellitesourceplugin/CMakeLists.txt:4
(qt_internal_add_plugin)

Pick-to: 6.11 6.12
Change-Id: I0002857608cedf347b84a2bf0c71cf01e748c535
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>