Add a test under ruby GC stress mode and fix non-GC-rooted variable bug
diff --git a/src/ruby/ext/grpc/rb_grpc.c b/src/ruby/ext/grpc/rb_grpc.c
index 933a3ac..b53f09e 100644
--- a/src/ruby/ext/grpc/rb_grpc.c
+++ b/src/ruby/ext/grpc/rb_grpc.c
@@ -315,8 +315,8 @@
     return;
   }
 
-  bg_thread_init_rb_mu = rb_mutex_new();
   rb_global_variable(&bg_thread_init_rb_mu);
+  bg_thread_init_rb_mu = rb_mutex_new();
 
   grpc_rb_mGRPC = rb_define_module("GRPC");
   grpc_rb_mGrpcCore = rb_define_module_under(grpc_rb_mGRPC, "Core");