add test for recent unwind UB issue

Author: RalfJung

tests/panic/oob_subslice.rs

@@ -0,0 +1,7 @@
+// This once failed with "unwinding past a stack frame that does not allow unwinding",
+// fixed by https://github.com/rust-lang/rust/issues/110233.
+
+fn main() {
+    let x = [1, 2, 3, 4];
+    let _val = &x[..=4];
+}

tests/panic/oob_subslice.stderr

@@ -0,0 +1,2 @@
+thread 'main' panicked at 'range end index 5 out of range for slice of length 4', $DIR/oob_subslice.rs:LL:CC
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace