ovn-northd.at: Fix flaky VXLAN mode disabling test.

This tests constructs a very large operation that frequently takes more
than a second on busy CI systems.  As a result ovn-sbctl emits a warning
for a long poll interval and fails the test.

Fix that by splitting a single large command into a series of smaller
ones - 100 switches at a time.  This makes the test a bit slower, but
much more reliable.

Fixes: 7e99500 ("northd: Add support for disabling vxlan mode.")
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

tests/ovn-northd.at

@@ -2882,12 +2882,16 @@ ovn-sbctl \
     --id=@e create encap chassis_name=hv1 ip="192.168.0.1" type="vxlan" \
     -- --id=@c create chassis name=hv1 encaps=@e
 
-cmd="ovn-nbctl --wait=sb"
+cmd=""
 for i in {1..4097..1}; do
     cmd="${cmd} -- ls-add lsw-${i}"
+    if test $(($i % 100)) -eq 0; then
+        check ovn-nbctl $cmd
+        cmd=""
+    fi
 done
 
-check $cmd
+check ovn-nbctl --wait=sb $cmd
 
 check_row_count nb:Logical_Switch 4097
 wait_row_count sb:Datapath_Binding 4095