test: add test that checks that --progress works correctly

This commit checks that the `--progress` argument generates the
expected output, i.e. with `term` we will not get any osbuild
output and the spinner. And with `verbose` we will not get a
spinner and the osbuild output.

test/conftest.py

@@ -32,6 +32,7 @@ def build_fake_container_fixture(tmpdir_factory, build_container):
     # do this images may fail with "broken" pipe errors
     cat - >/dev/null
 
+    echo "osbuild-stdout-output"
     mkdir -p /output/qcow2
     echo "fake-disk.qcow2" > /output/qcow2/disk.qcow2