[Buttons] Add setup and teardown to tests (#6432)
[Buttons] Add setup and teardown to tests (#6432)

## Related links
* Component: [MDCButton](https://github.com/material-components/material-components-ios/tree/develop/components/Buttons/src)
* Bug: This was discovered when working on #3408
* Related Apple docs: [`setUp & tearDown`](https://developer.apple.com/documentation/xctest/xctestcase/understanding_setup_and_teardown_for_test_methods)
## Introduction
While working on #6415 I noticed a lot of the tests within `ButtonTests.m` created an MDCButton in the given. This change implements a _setUp_ and _tearDown_ function that creates an MDCButton that all of the tests can then use.
## The problem
In almost every test we create a button and then test it. We can simply this by just doing a _setup_ and _teardown_.
## The fix
Add _setup_ and _teardown_ methods so that all the test that created a button in the _Given_ section didn't have to do that.
## Additional notes
Some of the test created a button and some created a button with a frame. If the frame had no effect on the test then setting the frame was removed.
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