test_out_exec_filter: add a sleep to ensure the stream is emitted (#4755)

**Which issue(s) this PR fixes**:
Fixes #4754

**What this PR does / why we need it**:
This PR will stable the tests.

**Docs Changes**:

**Release Note**:

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>
Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: Watson1978

test/plugin/test_out_exec_filter.rb

@@ -500,10 +500,18 @@ def create_driver(conf)
     d = create_driver(conf)
     time = event_time('2011-01-02 13:14:15')
 
-    d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: true,  shutdown: false){ d.feed(time, {"k1" => 0}) }
-    d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: false, shutdown: false){ d.feed(time, {"k1" => 1}) }
-    d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: false, shutdown: false){ d.feed(time, {"k1" => 2}) }
-    d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: false, shutdown: false){ d.feed(time, {"k1" => 3}) }
+    d.run(default_tag: 'test', expect_emits: 4) do
+      d.feed(time, {"k1" => 0})
+      d.flush
+      sleep 0.5
+      d.feed(time, {"k1" => 1})
+      d.flush
+      sleep 0.5
+      d.feed(time, {"k1" => 2})
+      d.flush
+      sleep 0.5
+      d.feed(time, {"k1" => 3})
+    end
 
     assert_equal "2011-01-02 13:14:15\ttest\t0\n", d.formatted[0]
     assert_equal "2011-01-02 13:14:15\ttest\t1\n", d.formatted[1]
@@ -524,9 +532,6 @@ def create_driver(conf)
     assert_equal pid_list[1], events[1][2]['child_pid']
     assert_equal pid_list[0], events[2][2]['child_pid']
     assert_equal pid_list[1], events[3][2]['child_pid']
-
-  ensure
-    d.run(start: false, shutdown: true)
   end
 
   # child process exits per 3 lines