add test for panicking attribute macros

Author: tshepang

tests/ui/proc-macro/custom-attr-panic.rs

@@ -0,0 +1,8 @@
+//@ aux-build: test-macros.rs
+
+extern crate test_macros;
+
+#[test_macros::panic_attr] //~ ERROR custom attribute panicked
+fn foo() {}
+
+fn main() {}

tests/ui/proc-macro/custom-attr-panic.stderr

@@ -0,0 +1,10 @@
+error: custom attribute panicked
+  --> $DIR/custom-attr-panic.rs:5:1
+   |
+LL | #[test_macros::panic_attr]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: message: panic-attr
+
+error: aborting due to 1 previous error
+