Avoid use of deprecated Lwt_main.yield

... by replacing them with the almost-equivalent `Lwt.pause`. Promises
constructed in this way have slightly different scheduling behaviour,
but this shouldn't impact the correctness of our code.

See ocsigen/lwt#855 for more details.

src/irmin-test/store.ml

@@ -1662,7 +1662,7 @@ module Make (S : Generic_key) = struct
 
   let rec write fn = function
     | 0 -> []
-    | i -> (fun () -> fn i >>= Lwt_unix.yield) :: write fn (i - 1)
+    | i -> (fun () -> fn i >>= Lwt.pause) :: write fn (i - 1)
 
   let perform l = Lwt_list.iter_p (fun f -> f ()) l
 
@@ -1751,7 +1751,7 @@ module Make (S : Generic_key) = struct
             let tag = Fmt.str "tmp-%d-%d" n i in
             let* m = S.clone ~src:t ~dst:tag in
             S.set_exn m ~info:(infof "update") (k i) (v i) >>= fun () ->
-            Lwt_unix.yield () >>= fun () ->
+            Lwt.pause () >>= fun () ->
             S.merge_into ~info:(infof "update: multi %d" i) m ~into:t
             >>= merge_exn "update: multi")
       in
@@ -1906,7 +1906,7 @@ module Make (S : Generic_key) = struct
                 let* m = S.clone ~src:t ~dst:tag in
                 S.set_exn m ~info:(infof "update") (k i) (v i) >>= fun () ->
                 let* set = S.Head.find m in
-                Lwt_unix.yield () >>= fun () -> S.Head.test_and_set t ~test ~set))
+                Lwt.pause () >>= fun () -> S.Head.test_and_set t ~test ~set))
       in
       let read t =
         read

src/irmin-test/store_watch.ml

@@ -22,7 +22,7 @@ module Make (Log : Logs.LOG) (S : Generic_key) = struct
 
   let sleep ?(sleep_t = 0.01) () =
     let sleep_t = min sleep_t 1. in
-    Lwt_unix.yield () >>= fun () -> Lwt_unix.sleep sleep_t
+    Lwt.pause () >>= fun () -> Lwt_unix.sleep sleep_t
 
   let now_s () = Mtime.Span.to_s (Mtime_clock.elapsed ())