filterx: add light tests for filterx isset/unset functions #1406

Job	Run time
create-source-tarball	1m 48s
create-packages / debian-bookworm	6m 26s
create-packages / debian-testing	6m 22s
create-packages / fedora-39	6m 20s
create-packages / ubuntu-jammy	8m 13s
	29m 9s

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