filterx: add light tests for filterx isset/unset functions #1407

Job	Run time
create-source-tarball	1m 52s
create-packages / debian-bookworm	6m 14s
create-packages / debian-testing	6m 21s
create-packages / fedora-39	6m 23s
create-packages / ubuntu-jammy	7m 38s
	28m 28s

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