Add support for running tests that are expected to fail

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

tests_errors/const-5.ispc

@@ -0,0 +1,6 @@
+// Can't assign to type
+
+int func() {
+    const int a[10] = {1,2,3,4,5,6,7,8,9,10};
+    a[0] = 1;
+}