Check for diagnostic messages on stderr, not stdout

[evan-goode]

For rpm-software-management/dnf5#1641.

Adds support for the <REPOSYNC> shorthand to Then stderr is and Then stderr matches line by line.

Introduces Then stdout contains lines matching and Then stderr contains lines matching steps which are similar to Then stdout contains lines but treat each expected line as a regex, like Then stdout matches line by line.

This patchset strengthens many tests; there were many instances of the pattern:

Then stdout is
     """
     <REPOSYNC>
     foo
     bar
     """

that were replaced with

Then stderr is
     """
     <REPOSYNC>
     """
 And stdout is
     """
     foo
     bar
     """

so now stderr is checked in many places where it wasn't before.

However, some tests were weakened, notably those following the pattern:

Then stderr is
     """
     No match for argument: nosuchpkg

     Warning: skipped PGP checks for 1 package from repository: dnf-ci-fedora-updates
     """

Because stderr now includes lots of other (non-error) information including progress bars for downloads and RPM transactions, these occurrences were changed to:

Then stderr contains lines
     """
     No match for argument: nosuchpkg
     Warning: skipped PGP checks for 1 package from repository: dnf-ci-fedora-updates
     """

We could maybe make <RPMDOWNLOAD> and <RPMTRANSACTION> markers that work similar to <REPOSYNC> to allow us to use Then stderr is again, but I'd rather do that in a follow-up issue.

These changes were made mostly manually with the help of Neovim macros.

[jan-kolarik]

/packit test rpm-software-management/dnf5#1641

[jan-kolarik]

LGTM, tests were run locally successfully. And thanks for adding some docs to the python functions here in the CI stack!