Pass busted the path to the detected Lua interpreter

Otherwise, busted may run a different interpreter than the one we
detected, without the capabilities we expect.  (For instance, we might
have detected the luajit interpreter, but busted might run the lua
interpreter, without the ffi module.)

CMakeLists.txt

@@ -501,6 +501,7 @@ if(BUSTED_PRG)
   add_custom_target(functionaltest
     COMMAND ${CMAKE_COMMAND}
       -DBUSTED_PRG=${BUSTED_PRG}
+      -DLUA_PRG=${LUA_PRG}
       -DNVIM_PRG=$<TARGET_FILE:nvim>
       -DWORKING_DIR=${CMAKE_CURRENT_SOURCE_DIR}
       -DBUSTED_OUTPUT_TYPE=${BUSTED_OUTPUT_TYPE}
@@ -514,6 +515,7 @@ if(BUSTED_PRG)
   add_custom_target(benchmark
     COMMAND ${CMAKE_COMMAND}
       -DBUSTED_PRG=${BUSTED_PRG}
+      -DLUA_PRG=${LUA_PRG}
       -DNVIM_PRG=$<TARGET_FILE:nvim>
       -DWORKING_DIR=${CMAKE_CURRENT_SOURCE_DIR}
       -DBUSTED_OUTPUT_TYPE=${BUSTED_OUTPUT_TYPE}
@@ -529,6 +531,7 @@ if(BUSTED_LUA_PRG)
   add_custom_target(functionaltest-lua
     COMMAND ${CMAKE_COMMAND}
       -DBUSTED_PRG=${BUSTED_LUA_PRG}
+      -DLUA_PRG=${LUA_PRG}
       -DNVIM_PRG=$<TARGET_FILE:nvim>
       -DWORKING_DIR=${CMAKE_CURRENT_SOURCE_DIR}
       -DBUSTED_OUTPUT_TYPE=${BUSTED_OUTPUT_TYPE}

cmake/RunTests.cmake

@@ -27,7 +27,7 @@ endif()
 
 execute_process(
   COMMAND ${BUSTED_PRG} ${TEST_TAG} ${TEST_FILTER} -v -o ${BUSTED_OUTPUT_TYPE}
-    --lazy --helper=${TEST_DIR}/${TEST_TYPE}/preload.lua
+    --lua=${LUA_PRG} --lazy --helper=${TEST_DIR}/${TEST_TYPE}/preload.lua
     --lpath=${BUILD_DIR}/?.lua ${TEST_PATH}
   WORKING_DIRECTORY ${WORKING_DIR}
   ERROR_VARIABLE err