bug: Don't create go.mod in testdata with -copy-parent-gomod

[tenntenn]

$ skeleton -copy-parent-gomod mytool
$ tree mytool 
mytool
├── cmd
│   └── mytool
│       └── main.go
├── go.mod
├── mytool.go
├── mytool_test.go
└── testdata
    └── src
        └── a
            ├── a.go
            └── go.mod <- THIS

6 directories, 6 files