CI: Try to fix an unstable test on macOS

Sometimes test_unwatched_files_should_be_removed is failed on CI.

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

test/plugin/test_in_tail.rb

@@ -1606,7 +1606,7 @@ def test_unwatched_files_should_be_removed
       d = create_driver(config, false)
       d.end_if { d.instance.instance_variable_get(:@tails).keys.size >= 1 }
       d.run(expect_emits: 1, shutdown: false) do
-        Fluent::FileWrapper.open("#{@tmp_dir}/tail.txt", "ab") { |f| f.puts "test3\n" }
+        Fluent::FileWrapper.open("#{@tmp_dir}/tail.txt", "ab") { |f| f.puts "test3\n"; f.fsync }
       end
 
       cleanup_file("#{@tmp_dir}/tail.txt")