test-support styles causes component tests to fail

I have a component needs to get the parent element's `height` and `width` during its `didInsertElement` phase, so I set up a test case renders a template snippet:

``` javascript
await render(hbs`

`);
```

then I got a weird result like below:


As we saw in this screenshot, the wrapper element renders only half of width and height compares to its styles, it causes the test to fail even its implementation is actually correct.

I found the root cause is the default styles in:

https://github.com/emberjs/ember-qunit/blob/633f7c2a44cc5559697ab767d59a91253e0fbe2c/vendor/ember-qunit/test-container-styles.css#L23-L25

but I'm not confident to remove it at all. I use this temporary solution currently:

``` javascript
await render(hbs`
// reset styles only for this test case:

#ember-testing { width: initial; height: initial; transform: initial; }

`);
```