test_aptpkg: Skip tests if /etc/apt/sources.list is missing
test_aptpkg: Skip tests if /etc/apt/sources.list is missing

If `/etc/apt/sources.list` does not exist, three functional test cases fail
with

```
FileNotFoundError: [Errno 2] No such file or directory: '/etc/apt/sources.list'
```

because they try to open this files:

```
________________________________ test_get_repos ________________________________

    def test_get_repos():
        """
        Test aptpkg.get_repos
        """
>       test_repo, comps = get_current_repo()

/<<PKGBUILDDIR>>/tests/pytests/functional/modules/test_aptpkg.py:109:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/<<PKGBUILDDIR>>/tests/pytests/functional/modules/test_aptpkg.py:64: in get_current_repo
    with salt.utils.files.fopen("/etc/apt/sources.list") as fp:
________________________ test_get_repos_multiple_comps _________________________

    def test_get_repos_multiple_comps():
        """
        Test aptpkg.get_repos when multiple comps
        exist in repo.
        """
>       test_repo, comps = get_current_repo(multiple_comps=True)

/<<PKGBUILDDIR>>/tests/pytests/functional/modules/test_aptpkg.py:126:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/<<PKGBUILDDIR>>/tests/pytests/functional/modules/test_aptpkg.py:64: in get_current_repo
    with salt.utils.files.fopen("/etc/apt/sources.list") as fp:
_____________________________ test_expand_repo_def _____________________________

    def test_expand_repo_def():
        """
        Test aptpkg.expand_repo_def when the repo exists.
        """
>       test_repo, comps = get_current_repo()

/<<PKGBUILDDIR>>/tests/pytests/functional/modules/test_aptpkg.py:169:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/<<PKGBUILDDIR>>/tests/pytests/functional/modules/test_aptpkg.py:64: in get_current_repo
    with salt.utils.files.fopen("/etc/apt/sources.list") as fp:
```

Some Debian buildd do not have `/etc/apt/sources.list` and therefore these test
cases fail:
https://buildd.debian.org/status/fetch.php?pkg=salt&arch=all&ver=3004%2Bdfsg1-9&stamp=1644870337&raw=0

So just skip these test cases if `/etc/apt/sources.list` does not exist.

Signed-off-by: Benjamin Drung <86a6db991ec5c1a96f98c92b98df9d547d7c8c84@ionos.com>

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt