main: push TestPrefixOArgs testcase struct into TestPrefixOArgs

No need to have it declared globally.

cli_args_test.go

@@ -5,18 +5,16 @@ import (
 	"testing"
 )
 
-type testcase struct {
+// TestPrefixOArgs checks that the "-o x,y,z" parsing works correctly.
+func TestPrefixOArgs(t *testing.T) {
+	testcases := []struct {
 		// i is the input
 		i []string
 		// o is the expected output
 		o []string
 		// Do we expect an error?
 		e bool
-}
-
-// TestPrefixOArgs checks that the "-o x,y,z" parsing works correctly.
-func TestPrefixOArgs(t *testing.T) {
-	testcases := []testcase{
+	}{
 		{
 			i: nil,
 			o: nil,