ExternalProject: Fix TEST_BEFORE_INSTALL for multi-config generators

In multi-config generators we must tell `ctest` what configuration to
test.

Reported-by: Taylor Braun-Jones <taylor@braun-jones.org>

Modules/ExternalProject.cmake

@@ -1240,6 +1240,9 @@ function(_ep_get_build_command name step cmd_var)
         if("x${step}x" STREQUAL "xTESTx")
           string(REGEX REPLACE "^(.*/)cmake([^/]*)$" "\\1ctest\\2" cmd "${cmd}")
           set(args "")
+          if(CMAKE_CONFIGURATION_TYPES)
+            list(APPEND args -C $<CONFIG>)
+          endif()
         endif()
       endif()
     else()