tests/ui: add minicore compiletest self-test

tests/ui/compiletest-self-test/minicore-smoke-test.rs

@@ -0,0 +1,20 @@
+//! Basic smoke test for `minicore` test auxiliary.
+//!
+//! This test is duplicated between ui/codegen/assembly because they have different runtest
+//! codepaths.
+
+//@ add-core-stubs
+//@ check-pass
+//@ compile-flags: --target=x86_64-unknown-linux-gnu
+//@ needs-llvm-components: x86
+
+#![crate_type = "lib"]
+#![feature(no_core)]
+#![no_std]
+#![no_core]
+
+extern crate minicore;
+use minicore::*;
+
+struct Meow;
+impl Copy for Meow {}