Fix Module.ExternalData test on VS 6

Run the test with the ctest --build-noclean option.  The test
CMakeLists.txt file already uses file(REMOVE_RECURSE) to clean out
downloaded data anyway.  Some file removed by "msdev ... /clean" causes
CMake to re-run in the middle of the build step and file(REMOVE_RECURSE)
wipes out already-generated files.

Tests/CMakeLists.txt

@@ -319,6 +319,7 @@ if(BUILD_TESTING)
     --build-generator ${CMAKE_TEST_GENERATOR}
     --build-project ExternalDataTest
     --build-makeprogram ${CMAKE_TEST_MAKEPROGRAM}
+    --build-noclean
     --force-new-ctest-process
     --test-command ${CMAKE_CTEST_COMMAND} -C \${CTEST_CONFIGURATION_TYPE} -V
     )