test: avoid conflict access to tempfile on windows

[kenhys]

Which issue(s) this PR fixes:

N/A

What this PR does / why we need it:

It is a straight forward enough approach to use File.stat for these
test cases.

In test case, setup creates a temporary file and each test case try to access
via Fluent::Filewrapper.stat but it causes INVALID_HANDLE_VALUE.

Docs Changes:

N/A

Release Note:

N/A

[kenhys]

On windows, it will pass.

> ruby -Ilib .\test\plugin\in_tail\test_position_file.rb
Loaded suite ./test/plugin/in_tail/test_position_file
Started
..............
Finished in 0.0289855 seconds.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
14 tests, 49 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
483.00 tests/s, 1690.50 assertions/s

[cosmo0920]

I'd confirmed that this patch makes green on position testcases on Windows.
@kenhys How long does it take to ready for review status?