test_in_tail: Make sure to wait completing first read

Otherwise whole of the first file might be detected as a rotated file so
that in_tail tries to read it from head despite of `read_from_head=false`

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

test/plugin/test_in_tail.rb

@@ -985,6 +985,7 @@ def sub_test_rotate_file(config = nil, expect_emits: nil, expect_records: nil, t
 
       d = create_driver(config)
       d.run(expect_emits: expect_emits, expect_records: expect_records, timeout: timeout) do
+        sleep(0.1) while d.instance.instance_variable_get(:@startup)
         size = d.emit_count
         file.puts "test3"
         file.puts "test4"