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.

middleware/dnssec/cache_test.go

@@ -9,8 +9,8 @@ import (
 )
 
 func TestCacheSet(t *testing.T) {
-	fPriv, rmPriv, _ := test.TempFile(t, ".", privKey)
-	fPub, rmPub, _ := test.TempFile(t, ".", pubKey)
+	fPriv, rmPriv, _ := test.TempFile(".", privKey)
+	fPub, rmPub, _ := test.TempFile(".", pubKey)
 	defer rmPriv()
 	defer rmPub()