JS: Add test for flow through Buffer.concat

This flow was lost since the existing model of concat() boxes its return value in ArrayElement. There is no explicit model of Buffer.concat.

javascript/ql/test/library-tests/TripleDot/buffer.js

@@ -0,0 +1,7 @@
+import 'dummy';
+
+function t1() {
+    const b1 = Buffer.from(source("t1.1"));
+    const b2 = Buffer.from(source("t1.2"));
+    sink(Buffer.concat([b1, b2]).toString("utf8")); // $ MISSING: hasTaintFlow=t1.1 hasTaintFlow=t1.2
+}