Use assert!(foo) instead of assert_eq!(foo, true)

Use assert!(foo) instead of assert_eq!(foo, true)

It is redundant to pass true/false to `assert_eq!` since `assert!`
already asserts true/false.

This may, however, be controversial if someone thinks that

```
    assert_eq!(foo, false);
```

is more clear than

```
    assert!(!foo);
```

Use `assert!` directly instead of `assert_eq!` with true/false argument.