Add a test for calling label.add() outside of vcl_init.

src/tests/label.vtc

@@ -96,6 +96,9 @@ varnish v1 -vcl {
                 elsif (req.url == "/outofrange") {
                         l.go(3);
                 }
+                elsif (req.url == "/add") {
+                        l.add(1, "VCL1");
+                }
         }
 
         sub vcl_backend_fetch {
@@ -127,6 +130,14 @@ client c1 {
         expect resp.http.VCL == <undef>
 } -run
 
+client c1 {
+        txreq -url "/add"
+        rxresp
+        expect resp.status == 503
+        expect resp.reason == "VCL failed"
+        expect resp.http.VCL == <undef>
+} -run
+
 client c1 {
         txreq
         rxresp
@@ -139,6 +150,7 @@ logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
         expect * * VCL_Error "^vmod dispatch error: l.go.-1.: n must be >= 0$"
         expect * * VCL_Error "^vmod dispatch error: l.go.1.: label 1 was not added$"
         expect * * VCL_Error "^vmod dispatch error: l.go.3.: highest label number is 2"
+        expect * * VCL_Error "^vmod dispatch error: l.add.1, VCL1. may only be called in vcl_init$"
         expect * * VCL_Error "^vmod dispatch error: l.go.0.: may only be called in client context, not in any vcl_backend_. subroutine$"
 } -start