Migrate run-make/const_fn_mir to rmake.rs

Author: GuillaumeGomez

src/tools/tidy/src/allowed_run_make_makefiles.txt

@@ -18,7 +18,6 @@ run-make/compiler-lookup-paths-2/Makefile
 run-make/compiler-lookup-paths/Makefile
 run-make/compiler-rt-works-on-mingw/Makefile
 run-make/compressed-debuginfo/Makefile
-run-make/const_fn_mir/Makefile
 run-make/crate-hash-rustc-version/Makefile
 run-make/crate-name-priority/Makefile
 run-make/cross-lang-lto-clang/Makefile

tests/run-make/const_fn_mir/Makefile

@@ -0,0 +1,8 @@
+// The `needs-unwind -Cpanic=abort` gives a different MIR output.
+
+use run_make_support::{cwd, diff, rustc};
+
+fn main() {
+    rustc().input("main.rs").emit("mir").output("dump-actual.mir").run();
+    diff().expected_file("dump.mir").actual_file("dump-actual.mir").run();
+}