Updated flexbox tests to better capture all 3 specs; fixes #812

Fixes #812.

Now have 3 flexbox tests:

```javascript
Modernizr.addTest('flexbox', testAllProps('flexBasis'));
```
```javascript
Modernizr.addTest('flexboxlegacy', testAllProps('boxDirection'));
```
```javascript
Modernizr.addTest('flexboxtweener', testAllProps('flexAlign'));
```

Uses new `testAllProps()` syntax so should probably wait until #933 is merged in.