free_cell_layout test fails on macOS

[RReverser]

Summary

Running cargo test on a library code with either stable or nightly Rust fails on macOS Mojave.

Steps to Reproduce

• First clone this repository that uses wee_alloc: ..............
• cd $REPO/wee_alloc
• cargo test

Actual Results

---- free_cell_layout stdout ----
thread 'free_cell_layout' panicked at 'assertion failed: `(left == right)`
  left: `Bytes(17)`,
 right: `Bytes(24)`: Safety and correctness depends on FreeCell being only one word larger than CellHeader', wee_alloc/src/lib.rs:345:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:211
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:221
   4: <std::panicking::begin_panic::PanicPayload<A> as core::panic::BoxMeUp>::get
             at src/libstd/panicking.rs:476
   5: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:390
   6: std::panicking::try::do_call
             at src/libstd/panicking.rs:325
   7: core::char::methods::<impl char>::escape_debug
             at src/libcore/panicking.rs:77
   8: wee_alloc::free_cell_layout
             at wee_alloc/src/lib.rs:345
   9: wee_alloc::free_cell_layout::{{closure}}
             at wee_alloc/src/lib.rs:344
  10: core::ops::function::FnOnce::call_once
             at /rustc/b68fc18c45350e1cdcd83cecf0f12e294e55af56/src/libcore/ops/function.rs:238
  11: <F as alloc::boxed::FnBox<A>>::call_box
             at src/libtest/lib.rs:1471
             at /rustc/b68fc18c45350e1cdcd83cecf0f12e294e55af56/src/libcore/ops/function.rs:238
             at /rustc/b68fc18c45350e1cdcd83cecf0f12e294e55af56/src/liballoc/boxed.rs:673
  12: panic_unwind::dwarf::eh::read_encoded_pointer
             at src/libpanic_unwind/lib.rs:102

Expected Results

All tests passing.

[RReverser]

Looks like the test itself is wrong (it says it expects size to be one word larger, but actually adds one byte).