test: Add a compile-fail test for attribute/syntax parsing

I don't currently know how to deal with syntax extensions that appear betweeen
an attribute and an item, so this test captures the error that occurs.

Issue #487

src/test/compile-fail/ext-after-attrib.rs

@@ -0,0 +1,8 @@
+// xfail-stage0
+// error-pattern:expecting \[, found fmt
+
+// Don't know how to deal with a syntax extension appearing after an
+// item attribute. Probably could use a better error message.
+#[foo = "bar"]
+#fmt("baz")
+fn main() { }