tests(CMakeLists): robustify against add'l versions of Boost (#2608)

* tests/CMakeLists: Robustify against add'l versions of Boost

* refactor: simpler impl
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

tests/CMakeLists.txt

@@ -226,12 +226,12 @@ find_package(Boost 1.56)
 
 if(Boost_FOUND)
   if(NOT TARGET Boost::headers)
+    add_library(Boost::headers IMPORTED INTERFACE)
     if(TARGET Boost::boost)
       # Classic FindBoost
-      add_library(Boost::headers ALIAS Boost::boost)
+      set_property(TARGET Boost::boost PROPERTY INTERFACE_LINK_LIBRARIES Boost::boost)
     else()
       # Very old FindBoost, or newer Boost than CMake in older CMakes
-      add_library(Boost::headers IMPORTED INTERFACE)
       set_property(TARGET Boost::headers PROPERTY INTERFACE_INCLUDE_DIRECTORIES
                                                   ${Boost_INCLUDE_DIRS})
     endif()