Move various stdlib tests to library/std/tests

library/std/tests/slice-from-array-issue-113238.rs

@@ -0,0 +1,8 @@
+// This intends to use the unsizing coercion from array to slice, but it only
+// works if we resolve `<&[u8]>::from` as the reflexive `From<T> for T`. In
+// #113238, we found that gimli had added its own `From<EndianSlice> for &[u8]`
+// that affected all `std/backtrace` users.
+#[test]
+fn slice_from_array() {
+    let _ = <&[u8]>::from(&[]);
+}