After a "bad collections", avoid switching back back to generational

After a major bad collection (one that collects too few objects),
next collection will be major again. In that case, avoid switching
back to generational mode (as it will have to switch again to
incremental to do next major collection).

testes/gc.lua

@@ -11,6 +11,12 @@ collectgarbage()
 
 local oldmode = collectgarbage("incremental")
 
+-- changing modes should return previous mode
+assert(collectgarbage("generational") == "incremental")
+assert(collectgarbage("generational") == "generational")
+assert(collectgarbage("incremental") == "generational")
+assert(collectgarbage("incremental") == "incremental")
+
 
 local function gcinfo ()
   return collectgarbage"count" * 1024