test: proceed with server on 'check_warnings' status

Before this patch luatest.server used to fail if the instance status
was different from 'ready'. Now it allows 'check_warnings' state too.
It is required to test legal corner cases inside config applier.

Part of #8967

NO_DOC=test helper update
NO_TEST=see NO_DOC
NO_CHANGELOG=see NO_DOC

test/luatest_helpers/server.lua

@@ -140,7 +140,8 @@ function Server:connect_net_box()
     local saved_eval = self.net_box.eval
     self.net_box.eval = function(self, expr, args, opts)
         if expr == 'return _G.ready' then
-            expr = "return require('config'):info().status == 'ready'"
+            expr = "return require('config'):info().status == 'ready' or " ..
+                          "require('config'):info().status == 'check_warnings'"
         end
         return saved_eval(self, expr, args, opts)
     end