Change the assert_eq message to be more verbose.

Closes #6221
fix test
auto merge of #7695 : cmr/rust/assert_eq_msg, r=pnkfelix

This changes it from 

```
left: true does not equal right: false
```

to

```
assertion failed: `(left == right) && (right == left)` (left: `true`, right: `false`)
```