CI: sys: quadlet %T test: do not rely on journal

Some people might expect this to work:

    systemctl --wait start foo
    journalctl -u foo  ---> displays output from foo

Well, it does not. Not reliably, anyway:

   https://github.com/systemd/systemd/issues/28650

Shrug, okay, deal with it: write value of %T to a tmpfile
instead of relying on journal. I tested with TMPDIR=<many values>
on an SELinux system and, by golly, it works fine.

Signed-off-by: Ed Santiago <santiago@redhat.com>

test/system/252-quadlet.bats

@@ -784,6 +784,7 @@ EOF
     #
     # Step 1: determine what systemd is using for %T. There does not
     # seem to be any systemctly way to find this.
+    percent_t_file="${PODMAN_TMPDIR}/foo"
     local service=get-percent-t.$(random_string 10).service
     local unitfile=${UNIT_DIR}/$service
     cat >$unitfile <<EOF
@@ -791,15 +792,12 @@ EOF
 Description=Get the value of percent T
 
 [Service]
-ExecStart=/bin/bash -c "echo --==%T==--"
+ExecStart=/bin/bash -c "echo %T >$percent_t_file"
+Type=oneshot
 EOF
     systemctl daemon-reload
     systemctl --wait start $service
-    echo "$_LOG_PROMPT journalctl -u $service"
-    run journalctl -u $service
-    echo "$output"
-    assert "$output" =~ " --==.*==--" "get-percent-T unit ran to completion"
-    percent_t=$(expr "$output" : ".* --==\(.*\)==--")
+    percent_t=$(< $percent_t_file)
     # Clean up. Don't bother to systemctl-reload, service_setup does that below.
     rm -f $unitfile