test: use socat for oomd prekill hooks

Arch rolling now ships Nmap 7.991, which contains Nmap commit
b408fc2, titled "Ignore stdin with recv-only"[1].

That change omits stdin from the Ncat descriptor list in recv-only
mode. For a Unix listener, the listening socket is then its sole
descriptor. Ncat enters its listen loop only when that list has more
than one descriptor or a connection already exists, so it exits
immediately after bind and leaves the socket pathname behind.

systemd-oomd sees that stale pathname, tries to invoke the hook, and
gets ECONNREFUSED. The notification output therefore remains empty
and the jq assertion fails. The problem is visible in trunk's CI
failures[2].

Use socat for these one-way Unix listeners. It remains in accept while
waiting for the hook and is already installed in integration images.

[1] https://github.com/nmap/nmap/commit/b408fc243ce26da7ca497595833c14ce36a5e1a4
[2] https://github.com/systemd/systemd/actions/runs/32622985037/job/97153925154

test/units/TEST-55-OOMD.sh

@@ -519,7 +519,7 @@ EOF
 
     # one hook
     mkdir -p /run/systemd/oomd.prekill.hook/
-    ncat --recv-only -kUl /run/systemd/oomd.prekill.hook/althook >/tmp/oomd_event.json &
+    socat -u UNIX-LISTEN:/run/systemd/oomd.prekill.hook/althook STDOUT >/tmp/oomd_event.json &
     ! systemctl start --wait TEST-55-OOMD-testbloat.service || exit 1
     [[ $(jq -r .method </tmp/oomd_event.json) = 'io.systemd.oom.Prekill.Notify' ]]
 
@@ -527,7 +527,7 @@ EOF
 
     # many hooks
     for i in {1..4}; do
-        ncat --recv-only -kUl "/run/systemd/oomd.prekill.hook/althook$i" >"/tmp/oomd_event$i.json" &
+        socat -u "UNIX-LISTEN:/run/systemd/oomd.prekill.hook/althook$i" STDOUT >"/tmp/oomd_event$i.json" &
     done
 
     ! systemctl start --wait TEST-55-OOMD-testbloat.service || exit 1