Fix unit test execution when building in shared linking mode

The unit test is built only when static linking, but the test
itself was still being invoked when running 'ctest' in shared
linking mode. This leads to the following test failure in 'ctest':

'Could not find executable vvenc_unit_test'.

This adds a fix so the unit test is not run in shared builds.

cmake/modules/vvencTests.cmake

@@ -19,7 +19,9 @@ add_test( NAME Test_vvenclibtest-sdk_default             COMMAND vvenclibtest 4
 add_test( NAME Test_vvenclibtest-sdk_stringapi_interface COMMAND vvenclibtest 5 )
 add_test( NAME Test_vvenclibtest-timestamps              COMMAND vvenclibtest 6 )
 
-add_test( NAME Test_vvenc_unit_test COMMAND vvenc_unit_test )
+if( NOT BUILD_SHARED_LIBS )
+  add_test( NAME Test_vvenc_unit_test COMMAND vvenc_unit_test )
+endif()
 
 set( CLEANUP_TEST_FILES "" )