Improve tests

Tests currently need to display themselves the result of their run like:

```js
print(`<${'='.repeat(level + 1)} "${suiteName}": ${x.getTotalErrors()} ` +
    `${plural('error', x.getTotalErrors())} (in ${x.getTotalRanTests()} ` +
    `${plural('test', x.getTotalRanTests())})`);
```

Instead, the test suite could handle it directly.