disable epoll1 by default (just enable tests)

src/core/lib/iomgr/ev_epoll1_linux.c

@@ -1050,6 +1050,10 @@ static const grpc_event_engine_vtable vtable = {
 /* It is possible that GLIBC has epoll but the underlying kernel doesn't.
  * Create a dummy epoll_fd to make sure epoll support is available */
 const grpc_event_engine_vtable *grpc_init_epoll1_linux(bool explicit_request) {
+  if (!explicit_request) {
+    return NULL;
+  }
+
   if (!grpc_has_wakeup_fd()) {
     return NULL;
   }

src/core/lib/iomgr/ev_posix.c

@@ -64,8 +64,8 @@ typedef struct {
 } event_engine_factory;
 
 static const event_engine_factory g_factories[] = {
-    {"epoll1", grpc_init_epoll1_linux},
     {"epollsig", grpc_init_epollsig_linux},
+    {"epoll1", grpc_init_epoll1_linux},
     {"epoll-threadpool", grpc_init_epoll_thread_pool_linux},
     {"epoll-limited", grpc_init_epoll_limited_pollers_linux},
     {"poll", grpc_init_poll_posix},