Make ExportLanguages a subtest of the ObjectLibrary test

It was a subtest of the RunCMake.ObjectLibrary test. However, we need
to test a build with ExternalProject after running CMake, which RunCMake tests
do not do.

Tests/ObjectLibrary/ExportLanguages/ExportLanguagesTest/CMakeLists.txt

@@ -0,0 +1,14 @@
+
+cmake_minimum_required(VERSION 2.8)
+
+project(ExportLanguagesTest)
+
+include(${CMAKE_CURRENT_BINARY_DIR}/../BExport.cmake)
+get_property(configs TARGET ExpExportLanguagesB PROPERTY IMPORTED_CONFIGURATIONS)
+foreach(c ${configs})
+  get_property(langs TARGET ExpExportLanguagesB PROPERTY IMPORTED_LINK_INTERFACE_LANGUAGES_${c})
+  list(FIND langs CXX pos)
+  if(${pos} LESS 0)
+    message(FATAL_ERROR "Target export does not list object library languages.")
+  endif()
+endforeach()