tests: removed code duplication (#14596)

- [x] Factored to a common place the fixture `testing_gpg_directory`, renamed it as 
      `mock_gnupghome`
- [x] Removed altogether the function `has_gnupg2`

For `has_gnupg2`, since we were not trying to parse the version from the output of:
```console
$ gpg2 --version
```
this is effectively equivalent to check if `spack.util.gpg.GPG.gpg()` was found. If we need to ensure version is `^2.X` it's probably better to do it in `spack.util.gpg.GPG.gpg()` than in a separate function.