Avoid a race by using syncvsl

Without syncvsl, client N+1 may emitting VSL before client N
has flushed its VSL.

(In general we should probably always use syncvsl with logexpect)

bin/varnishtest/tests/m00049.vtc

@@ -6,6 +6,7 @@ varnishtest "VMOD blob workspace overflow conditions"
 # will not result in a compilation failure.
 
 varnish v1 -cliok "param.set vcc_err_unref off"
+varnish v1 -cliok "param.set debug +syncvsl"
 
 shell {
 	cat >vrt_blob.vcl <<-EOF