test: fix replication/gh-5806-xlog-cleanup potential flakiness

Follow-up #6329

test/replication/gh-5806-xlog-cleanup.result

@@ -83,6 +83,10 @@ test_run:switch('replica')
  | ---
  | - true
  | ...
+test_run:wait_upstream(1, {status = 'follow'})
+ | ---
+ | - true
+ | ...
 box.cfg{checkpoint_count = 1}
  | ---
  | ...
@@ -252,6 +256,10 @@ test_run:switch('replica')
  | ---
  | - true
  | ...
+test_run:wait_upstream(1, {status = 'follow'})
+ | ---
+ | - true
+ | ...
 box.cfg{checkpoint_count = 1}
  | ---
  | ...

test/replication/gh-5806-xlog-cleanup.test.lua

@@ -45,6 +45,7 @@ test_run:cmd('start server replica')
 -- there gonna be an own data loss). This allows us to
 -- trigger XlogGapError in the log.
 test_run:switch('replica')
+test_run:wait_upstream(1, {status = 'follow'})
 box.cfg{checkpoint_count = 1}
 s = box.schema.space.create('testreplica')
 _ = s:create_index('pk')
@@ -122,6 +123,7 @@ test_run:cmd('create server replica with rpl_master=master,\
 test_run:cmd('start server replica')
 
 test_run:switch('replica')
+test_run:wait_upstream(1, {status = 'follow'})
 box.cfg{checkpoint_count = 1}
 s = box.schema.space.create('testreplica')
 _ = s:create_index('pk')