In the mmdebstrap autpkgtest, debian/tests/sourcesfilter will write out
/etc/apt/sources.list before the test's setup-hook is run. Thus, the
test's setup-hook will overwrite the contents of the sources.list that
debian/tests/sourcesfilter set up, which will end up pulling in the
wrong packages.
Thus, only write out our own /etc/apt/sources.list if it does not yet
exist. If it does exist, nothing needs to be done.
