fixing qcor ci fail, uncomment out xacc build test conditional in aer plugin

Signed-off-by: Alex McCaskey <mccaskeyaj@ornl.gov>

quantum/plugins/ibm/aer/CMakeLists.txt

@@ -42,9 +42,9 @@ else()
   set_target_properties(${LIBRARY_NAME} PROPERTIES LINK_FLAGS "-shared")
 endif()
 
-#if(XACC_BUILD_TESTS)
+if(XACC_BUILD_TESTS)
   add_subdirectory(tests)
-#endif()
+endif()
 
 install(TARGETS ${LIBRARY_NAME} DESTINATION ${CMAKE_INSTALL_PREFIX}/plugins)