test: fix replication/errinj potential flakiness

The test has a function wait_repl() which tries to wait until the data
is replicated to the other node for a tiny timeout (0.2 seconds)

This didn't cause issues in the past, but it became an issue after
applier in thread introduction.

Let's use the default test_run:wait_cond() approach instead of a custom
one. This way we increase waiting timeout (making the test pass with a
higher chance).

Follow-up #6329

test/replication/errinj.result

@@ -43,13 +43,7 @@ test_run:cmd("setopt delimiter ';'")
 ...
 -- vinyl does not support index.len() so we use index.count() instead
 function wait_repl(cnt)
-    for i = 1, 20 do
-        if s.index[0]:count() >= cnt then
-            return true
-        end
-        fiber.sleep(0.01)
-    end
-    return false
+    return test_run:wait_cond(function() return s.index[0]:count() >= cnt end)
 end;
 ---
 ...

test/replication/errinj.test.lua

@@ -18,13 +18,7 @@ s = box.space.test
 test_run:cmd("setopt delimiter ';'")
 -- vinyl does not support index.len() so we use index.count() instead
 function wait_repl(cnt)
-    for i = 1, 20 do
-        if s.index[0]:count() >= cnt then
-            return true
-        end
-        fiber.sleep(0.01)
-    end
-    return false
+    return test_run:wait_cond(function() return s.index[0]:count() >= cnt end)
 end;
 test_run:cmd("setopt delimiter ''");