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/proxy_test.go

@@ -21,7 +21,7 @@ example.org.		IN	A	127.0.0.2
 `
 
 func TestLookupProxy(t *testing.T) {
-	name, rm, err := test.TempFile(t, ".", exampleOrg)
+	name, rm, err := test.TempFile(".", exampleOrg)
 	if err != nil {
 		t.Fatalf("failed to created zone: %s", err)
 	}