Adjust tests/ui/attr-shebang.rs

- Move `tests/ui/attr-shebang.rs` to `tests/ui/attributes/`.
- Downgrade test to `check-pass`, this would fail very early if the
  parser did not accept `#![..]` attributes.
- Briefly document test intent.

tests/ui/attr-shebang.rs

@@ -1,5 +0,0 @@
-//@ run-pass
-
-#![allow(stable_features)]
-#![feature(rust1)]
-pub fn main() { }

tests/ui/attributes/attr-shebang.rs

@@ -0,0 +1,7 @@
+//! Check that we accept crate-level inner attributes with the `#![..]` shebang syntax.
+
+//@ check-pass
+
+#![allow(stable_features)]
+#![feature(rust1)]
+pub fn main() { }