Prevent some race condition making test fail sometime

Saving the webhook to /tmp was sometimes not finished before curl was caused,
resulting in an empty payload -> validation error
Passing via STDIN could be an alternative...

tests/helper.bash

@@ -23,6 +23,8 @@ function gen_webhook() {
 function send_webhook {
     TMPFILE="$(mktemp)"
     echo -n "$2" > $TMPFILE
+    # To prevent race conditions where curl is started while $TMPFILE is still empty
+    sync
 
     # We can make a few attempts, just in case the webserver hasn't started yet
     n=0