Test gdb pretty printing more and fix overzealous type substitution
Adresses a problem concerning printing BTreeMap / BTreeSet data in gdb: when the key or value type name contains substring "LeafNode", and the map has multiple nodes (e.g. more than 11 elements), printing causes an exception. E.g.
```
rustc -g - <<EOF
use std::collections::BTreeMap;
struct MyLeafNode(i8);
fn main() {
let m: BTreeMap<i8, MyLeafNode> = (0..12).map(|i| (i, MyLeafNode(i))).collect();
assert!(!m.is_empty());
}
EOF
```
```
$ rust-gdb rust_out
(gdb) b 7
(gdb) r
(gdb) p m
$1 = BTreeMap<i8, rust_out::MyLeafNode>(len: 12)Python Exception <class 'gdb.error'> No type named alloc::collections::btree::node::InternalNode<i8, rust_out::MyInternalNode>.:
use std::collections::BTreeMap;
```
The code was written in #56144 by @tromey (and later touched upon by @RalfJung in #57045, but I think that had nothing to do with the issues in this PR).