udevadm-test: show result in json format (#36284)

This adds `--json=MODE` command line option to `udevadm test`.
This may be useful for parsing e.g. network interface name or device
node symlinks.

Closes #23661.

test/units/TEST-17-UDEV.10.sh

@@ -153,6 +153,10 @@ udevadm test -N late /sys/class/net/$netdev
 udevadm test --resolve-names never /sys/class/net/$netdev
 (! udevadm test -N hello /sys/class/net/$netdev)
 udevadm test -v /sys/class/net/$netdev
+udevadm test --json=off /sys/class/net/$netdev
+udevadm test --json=pretty /sys/class/net/$netdev | jq . >/dev/null
+udevadm test --json=short /sys/class/net/$netdev | jq . >/dev/null
+udevadm test --json=help
 udevadm test -h
 
 # udevadm test-builtin path_id "$loopdev"