test: refactor WPTRunner

- Print test results as soon as they are available, instead of
  until after all the tests are complete. This helps us printing
  tests whose completion callback is not called because of
  failures.
- Run the scripts specified by `// META: script=` one by one
  instead of concatenating them first for better error stack
  traces.
- Print a status summary when the test process is about to exit.
  This can be used as reference for updating the status file.

For example the stderr output of
`out/Release/node test/wpt/test-console.js` would be:

```
{
  'idlharness.any.js': {
    fail: {
      expected: [
        'assert_equals: operation has wrong .length expected 1 but got 0'
      ]
    }
  }
}
Ran 4/4 tests, 0 skipped, 3 passed, 1 expected failures, 0 unexpected failures
```