test: cherry-pick fix for memtx_gc_after_snapshot_test

This is part of master commit bd4c6675 ("memtx: use MemtxAllocator
stats for box.info.memory"). The test is failed without the patch in
ASAN build.

Follow-up #7327

NO_CHANGELOG=test fix
NO_DOC=test fix

test/box-luatest/memtx_gc_after_snapshot_test.lua

@@ -31,6 +31,7 @@ g.test_memtx_gc_after_snapshot = function(cg)
             box.space.test:delete(i)
         end
         box.commit()
+        box.tuple.new() -- drop blessed tuple ref
         collectgarbage('collect') -- drop Lua refs
         local mem_used_3 = box.info.memory().data
         t.assert_gt(mem_used_2 - mem_used_3, 1000000)