ci: run unit tests on Debian testing again #222

Job	Run time
install (debian:bullseye-slim)	16s
install (debian:bookworm-slim)	13s
install (debian:testing-slim)	14s
yamllint	11s
system-testing (debian:unstable-slim)	4m 7s
install (ubuntu:jammy)	26s
unittest (debian:bullseye-slim)	30s
system-testing (ubuntu:jammy)	3m 43s
unittest (debian:bookworm-slim)	30s
install (ubuntu:mantic)	20s
system-testing (ubuntu:mantic)	4m 20s
unittest (debian:testing-slim)	33s
unittest (debian:unstable-slim)	35s
unittest (ubuntu:jammy)	32s
unittest (ubuntu:mantic)	30s
	17m 0s

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