add tests for the sub() method and function to vtc's adapted from

Varnish regression tests

src/tests/r00896.vtc

@@ -30,10 +30,17 @@ varnish v1 -vcl+backend {
 		return(synth(500, "not ok"));
             }
 	}
+
+	sub vcl_deliver {
+	    set resp.http.long
+	        = re2.sub("^(abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij|abcdefghij)",
+	                  req.http.host, "z", "fallback");
+	}
 } -start 
 
 client c1 {
 	txreq  
 	rxresp
 	expect resp.status == 200
+	expect resp.http.long == "fallback"
 } -run

src/tests/r00913.vtc

@@ -42,6 +42,9 @@ varnish v1 -vcl+backend {
 		if (re2.match("", beresp.http.foo)) {
 			set beresp.http.barf = "YYY";
 		}
+		# Tests for sub() with regex "$" are in sub.vtc
+		set beresp.http.barsub = empty.sub(beresp.http.foo, "YYY");
+		set beresp.http.barsubf = re2.sub("", beresp.http.foo, "YYY");
 	}
 } -start
 
@@ -55,4 +58,6 @@ client c1 {
 	expect resp.http.bars == "YYY"
 	expect resp.http.foof == "XXX"
 	expect resp.http.barf == "YYY"
+	expect resp.http.barsub == "YYYXXX"
+	expect resp.http.barsubf == "YYYXXX"
 } -run