flake-parts: add `devenv test` support #3184

Job	Run time
generate-examples	19s
build (self-hosted, linux, X64)	1m 32s
build (macos-13)	12m 34s
build (self-hosted, macOS, ARM64)	3m 40s
build (self-hosted, linux, ARM64)	2m 6s
tests	0s
zsh/fish (${{ join(matrix.os) }})	0s
direnv (${{ join(matrix.os) }})	0s
${{ matrix.example }} (${{ join(matrix.os) }})	0s
pin	0s
	20m 11s

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