storage: deflake TestStoreRangeMergeWatcher

This test could deadlock if the LHS replica on store2 was shut down
before it processed the split at "b". Teach the test to wait for the LHS
replica on store2 to process the split before blocking Raft traffic to
it.

Fixes #31096.
Fixes #31149.
Fixes #31160.
Fixes #31167.

Release note: None

pkg/storage/client_merge_test.go

@@ -2032,12 +2032,19 @@ func testMergeWatcher(t *testing.T, injectFailures bool) {
 	}
 	mtc.transferLease(ctx, rhsDesc.RangeID, 0, 2)
 
-	// Block Raft traffic to the LHS replica on store2, by holding its raftMu, so
-	// that its LHS isn't aware there's a merge in progress.
+	// After the LHS replica on store2 processes the split, block Raft traffic to
+	// it by holding its raftMu, so that it isn't aware there's a merge in
+	// progress.
 	lhsRepl2, err := store2.GetReplica(lhsDesc.RangeID)
 	if err != nil {
 		t.Fatal(err)
 	}
+	testutils.SucceedsSoon(t, func() error {
+		if !lhsRepl2.Desc().Equal(lhsDesc) {
+			return errors.New("store2 has not processed split")
+		}
+		return nil
+	})
 	lhsRepl2.RaftLock()
 
 	args := adminMergeArgs(lhsDesc.StartKey.AsRawKey())