The passthrough_env function was unnecessarily complicated,
hence fragile. Clean it up, and add regression tests.
For future reference: CI broke horribly because of this.
Rootless tests all failed with missing CI_DESIRED_NETWORK.
Root cause was that CIRRUS_CHANGE_TITLE had a trailing
space which, because of shell indirection, passthrough_env()
wrote as trailing backslash (not backslash-space) in the
/etc/ci_environment file, which then caused the next line
in the file to get glommed onto CIRRUS_CHANGE_TITLE.
Signed-off-by: Ed Santiago <santiago@redhat.com>
bdd5f82458