Add variable-length array trace test

Without the fix in 3ccb482 this
produces a trace containing nil expressions; with it, they are
successfully evaluated constant expresssions as expected.

Author: smowton

regression/cbmc-java/VarLengthArrayTrace1/Container.class

@@ -0,0 +1,21 @@
+public class VarLengthArrayTrace1 {
+
+  public static void main(int unknown, int arg2) {
+
+    Container c = new Container(unknown);
+    if(c.val != 2) return;
+    int[] arr = new int[c.val];
+    arr[1] = arg2;
+    int test = arr[1];
+    assert(test != 10);
+
+  }
+
+}
+
+class Container {
+
+  public Container(int v) { val = v; }
+  int val;
+
+}

regression/cbmc-java/VarLengthArrayTrace1/VarLengthArrayTrace1.class

@@ -0,0 +1,10 @@
+CORE
+VarLengthArrayTrace1.class
+--trace
+^EXIT=10$
+^SIGNAL=0$
+dynamic_3_array\[1l\]=10
+--
+^warning: ignoring
+assignment removed
+irep("(\\"nil\\")")