Add test of r#Backtrace that is not std::backtrace::Backtrace

[dtolnay]

Without r#Backtrace:

error[E0308]: mismatched types
   --> tests/test_backtrace.rs:42:14
    |
42  |     #[derive(Error, Debug)]
    |              ^^^^^
    |              |
    |              expected `std::backtrace::Backtrace`, found `not_backtrace::Backtrace`
    |              arguments to this method are incorrect
    |
    = note: `not_backtrace::Backtrace` and `std::backtrace::Backtrace` have similar names, but are actually distinct types
note: `not_backtrace::Backtrace` is defined in the current crate
   --> tests/test_backtrace.rs:26:9
    |
26  |         pub struct Backtrace;
    |         ^^^^^^^^^^^^^^^^^^^^
note: `std::backtrace::Backtrace` is defined in crate `std`
   --> $RUSTUP_HOME/toolchains/nightly-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/backtrace.rs:108:1
    |
108 | pub struct Backtrace {
    | ^^^^^^^^^^^^^^^^^^^^
note: method defined here
   --> $RUSTUP_HOME/toolchains/nightly-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/error.rs:607:12
    |
607 |     pub fn provide_ref<T: ?Sized + 'static>(&mut self, value: &'a T) -> &mut Self {
    |            ^^^^^^^^^^^
    = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)