test(styles): update button styles for test pages (#29931)

Issue number: internal

---------

<!-- Please do not submit updates to dependencies unless it fixes an
issue. -->

<!-- Please try to limit your pull request to one type (bugfix, feature,
etc). Submit multiple pull requests if needed. -->

## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

Test styles causes native buttons to have [certain
styling](https://github.com/ionic-team/ionic-framework/blob/main/core/scripts/testing/styles.css#L52-L64).
This was done to spruce up the buttons used for testing purposes only.
However, this ended up adding styles to native buttons within Ionic
components.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Test styles for native buttons are only applied to buttons that are
not part of a Ionic component

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

<!--
  If this introduces a breaking change:
1. Describe the impact and migration path for existing applications
below.
  2. Update the BREAKING.md file with the breaking change.
3. Add "BREAKING CHANGE: [...]" to the commit description when merging.
See
https://github.com/ionic-team/ionic-framework/blob/main/docs/CONTRIBUTING.md#footer
for more information.
-->


## Other information

<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->

How to test:

1. Run the project locally from the `main` branch
2.
[Comment](https://github.com/ionic-team/ionic-framework/blob/5d208e9daacad63294489e0b1bd6ec2b2645babd/core/src/components/searchbar/searchbar.md.scss#L91)
out `border: 0` from `.searchbar-clear-button` from the searchbar `md`
theme file (`ios` also works)
3. Navigate to the basic test page:
`/src/components/searchbar/test/basic`
4. Notice a teal border around the clear buttons
5. Checkout to this PR's branch
6. Make sure steps 2-3 are done
7. Verify that the teal border is not being applied to the clear buttons
8. Verify that only native buttons outside of the Ionic components have
a teal appearance: `/src/components/loading/test/standalone` and
`/src/components/action-sheet/test/is-open`