cmake build does not provide QC test dependencies

for qmake based builds there's QTC_TEST_DEPENDS, for Qbs based builds there's a pluginTestDepends property, but there's nothing similar for cmake based builds

this allows to specify dependencies for the runtime of the integrated plugin unit tests which are no hard dependency

(compare e.g. running AutoTest or ClangTools plugin unit tests just by passing

-test PluginName

both are testing the plugin, but some tests fail due to not loading QbsProjectManager which is not a hard dependency for these, but they should get loaded when the tests get executed)