* Add a setup test for middleware/file

This fix adds a setup test for middleware/file so that there is
a basic coverage for the Corefile processing of middleware/file.

This fix is related to 308 (Will look into it).
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* middleware/file: use helper function for test

Fixup setup_test.go and use the test.TempFile function to make things
somewhat shorter.

Use clean up the use of testing.T in TempFile - it is not used.