test: Test multiline headers in hook output

Hook output is checked to see if it looks like a header, which includes
the possibility of multi-line headers.

This patch extends the tests to include a multi-line header, to prevent
accidental regressions.

test/t-10-hooks/config/hooks/post-data.good

@@ -16,4 +16,6 @@ if [ "$RCPT_TO" == "permanent@testserver" ]; then
 fi
 
 echo "X-Post-Data: success"
+echo "X-Post-Data-Multiline: multiline"
+echo "  header for testing."