test: wait for the swtpm socket to appear

On slower/overloaded systems it may take a bit for the swtpm socket
to show up:

I: Started swtpm as PID 189419 with state dir /tmp/tmp.pWqUutuGUj
I: Configured emulated TPM2 device tpm-spapr
+ tee /var/tmp/systemd-test-TEST-70-TPM2_1/console.log
+ timeout --foreground 1200 /bin/qemu-system-ppc64le -smp 4 ...
qemu-system-ppc64le: -chardev socket,id=chrtpm,path=/tmp/tmp.pWqUutuGUj/sock: Failed to connect to '/tmp/tmp.pWqUutuGUj/sock': No such file or directory
E: qemu failed with exit code 1

Spotted regularly in the ppc64le cron job and in some Ubuntu CI/CentOS CI
pr runs [0].

[0] systemd/systemd#29183 (comment)

(cherry picked from commit 18c3ffb)
(cherry picked from commit 2171f68)
(cherry picked from commit bf449d7)

Author: mrc0mmand

test/test-functions

@@ -369,7 +369,12 @@ qemu_setup_swtpm_socket() {
     swtpm socket --tpm2 --tpmstate dir="$state_dir" --ctrl type=unixio,path="$state_dir/sock" &
     pid=$!
     if ! kill -0 "$pid"; then
-        echo >&2 "Failed to setup swtpm socket"
+        derror "Failed to start swtpm"
+        return 1
+    fi
+
+    if ! timeout 5 bash -c "until [[ -S $state_dir/sock ]]; do sleep .5; done"; then
+        derror "Failed to setup swtpm socket"
         return 1
     fi