fix unrelated syntax errors in vtcs.

These tests had syntax errors besides the error handling actually being
tested, which triggered with future changes to VCC.

Originally from #3163

bin/varnishtest/tests/e00030.vtc

@@ -2,6 +2,8 @@ varnishtest "Test req_top.* in an ESI context"
 
 varnish v1 -arg "-p feature=+esi_disable_xml_check" \
 	   -errvcl {Variable is read only.} {
+	backend foo None;
+
 	sub vcl_recv {
 		set req_top.url = "/foo";
 	}

bin/varnishtest/tests/v00018.vtc

@@ -9,7 +9,7 @@ varnish v1 -vcl {
 
 varnish v1 -errvcl {Variable is read only.} {
 	backend b { .host = "${localhost}"; }
-	sub vcl_miss { set now = 1000; }
+	sub vcl_miss { set now += 1s; }
 }
 
 varnish v1 -vcl {