Restore regression test for #2184

Introduced in 7c9916a6, its precise syntax got lost in polishing.

bin/varnishtest/tests/v00020.vtc

@@ -83,6 +83,7 @@ varnish v1 -vcl {
 		set req.http.foo = 1 - 1;
 		set req.http.foo = 1 + -1;
 		set req.http.foo = 1 - -1;
+		set req.http.foo = 1- -1; # regression test for #2184
 		set req.http.foo = 3 * 2;
 		set req.http.foo = 3 / 2;
 		set req.http.foo = 3 * -2;