test: fix flaky router_test

router_test.lua fails constantly on integration CI in tarantool
with error 'Peer closed' after simulating unconfigured box.

The reason for this problem is unclear, but the following changes
fix it. It might be related to the code, luatest executes
between execs (e.g. type(box.cfg) or box.cfg.<key> = ...).

Closes #403

NO_DOC=testfix

test/router-luatest/router_test.lua

@@ -469,24 +469,23 @@ g.test_enable_disable = function(g)
         _G.fiber_new:set_joinable(true)
     end, {global_cfg})
 
-    -- emulate unconfigured box
-    router:exec(function()
-        rawset(_G, 'old_box_cfg', box.cfg)
-        box.cfg = function(...) return _G.old_box_cfg(...) end
-    end)
-
     local err1, err2 = router:exec(function()
+        -- emulate unconfigured box
+        local old_box_cfg = box.cfg
+        box.cfg = function(...) return old_box_cfg(...) end
+
         rawset(_G, 'static_router', ivshard.router.internal.routers._static_router)
         rawset(_G, 'new_router', ivshard.router.internal.routers.new_router)
         local _, err_1 = pcall(_G.static_router.info, _G.static_router)
         local _, err_2 = pcall(_G.new_router.info, _G.new_router)
+
+        box.cfg = old_box_cfg
         return err_1, err_2
     end)
     assert_errors_equals(err1, err2, 'box seems not to be configured')
 
     -- set box status to loading
     router:exec(function()
-        box.cfg = _G.old_box_cfg
         rawset(_G, 'old_box_info', box.info)
         box.info = {status = 'loading'}
     end)