Pr 311 2 (#312)

* 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.

test/file_test.go

@@ -3,7 +3,7 @@ package test
 import "testing"
 
 func TestTempFile(t *testing.T) {
-	_, f, e := TempFile(t, ".", "test")
+	_, f, e := TempFile(".", "test")
 	if e != nil {
 		t.Fatalf("failed to create temp file: %s", e)
 	}