dnf seems to have changed the default for skip_if_unavailable. Some
mock repositories are still around in later tests, which then fail
because metadata cannot be synced.
Also expose skip_if_unavailable in dnf_repo_to_file_repo(), so that
tests checking for equality of repo files continue to pass.
In some cases when the host has, for whatever reason, multiple copies of
the same repo listed the build may fail with an error about running out
of space.
So this commit removes duplicate entries after the host's repos have been
loaded. It also adjusts some of the test repos to use different
temporary repo names for the tests.
