Add support for running tests that are expected to fail

Also add should-fail tests that exercise const and decl
initializers

tests_errors/decl-1.ispc

@@ -0,0 +1,5 @@
+// requires 10 values; 11 provided
+
+int func() {
+    int a[10] = {1,2,3,4,5,6,7,8,9,10,11};
+}