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

Issue number: internal

---------

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## 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

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## Other information

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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`

Author: thetaPC

core/scripts/testing/styles.css

@@ -16,7 +16,7 @@
       ion-app > ion-content {
         --background: #dddddd;
       }
-      ion-content button {
+      ion-content button.trigger {
         padding: 12px 16px;
       }
       .grid {
@@ -57,26 +57,30 @@
         <div class="grid">
           <div class="grid-item">
             <h2>Cover</h2>
-            <button id="cover-trigger">Trigger</button>
+            <button id="cover-trigger" class="trigger">Trigger</button>
             <ion-popover show-backdrop="false" class="cover-popover" trigger="cover-trigger" size="cover">
               <ion-content class="ion-padding"> My really really really really long content </ion-content>
             </ion-popover>
           </div>
           <div class="grid-item">
             <h2>With Event</h2>
-            <button id="event-trigger" onclick="openPopover('event-popover', event, 'false')">Trigger</button>
+            <button id="event-trigger" class="trigger" onclick="openPopover('event-popover', event, 'false')">
+              Trigger
+            </button>
           </div>
 
           <div class="grid-item">
             <h2>Auto</h2>
-            <button id="auto-trigger">Trigger</button>
+            <button id="auto-trigger" class="trigger">Trigger</button>
             <ion-popover show-backdrop="false" class="auto-popover" trigger="auto-trigger">
               <ion-content class="ion-padding"> My really really really really long content </ion-content>
             </ion-popover>
           </div>
           <div class="grid-item">
             <h2>No Event</h2>
-            <button id="no-event-trigger" onclick="openPopover('no-event-popover', null, 'true')">Trigger</button>
+            <button id="no-event-trigger" class="trigger" onclick="openPopover('no-event-popover', null, 'true')">
+              Trigger
+            </button>
           </div>
         </div>
       </ion-content>