Add support for running tests that are expected to fail

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

tests_errors/const-3.ispc

@@ -0,0 +1,9 @@
+// Can't assign to type
+
+struct Foo {
+    int x;
+};
+
+int func(const Foo f) {
+    f.x = 0;
+}