tests: Verify traits with unsupported ABIs must error

Author: workingjubilee

tests/ui/abi/unsupported-trait-decl.rs

@@ -0,0 +1,11 @@
+//@ compile-flags: --crate-type=lib
+
+#![feature(abi_gpu_kernel)]
+
+trait T {
+    extern "gpu-kernel" fn mu();
+    //~^ ERROR[E0570]
+}
+
+type TAU = extern "gpu-kernel" fn();
+//~^ ERROR[E0570]

tests/ui/abi/unsupported-trait-decl.stderr

@@ -0,0 +1,15 @@
+error[E0570]: `"gpu-kernel"` is not a supported ABI for the current target
+  --> $DIR/unsupported-trait-decl.rs:6:12
+   |
+LL |     extern "gpu-kernel" fn mu();
+   |            ^^^^^^^^^^^^
+
+error[E0570]: `"gpu-kernel"` is not a supported ABI for the current target
+  --> $DIR/unsupported-trait-decl.rs:10:19
+   |
+LL | type TAU = extern "gpu-kernel" fn();
+   |                   ^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0570`.