docs(testing): improve standalone testing documentation

[Zac-Smucker-Bryan]

Issue number: resolves #31312

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What is the current behavior?

Current docs don't provide recommended testing guidance for accurately testing standalone components. This can create issues because a standalone (custom-elements) build doesn't expose el.componentOnReady(), so any test that reads fixture.nativeElement before hydration in a standalone project has the potential to not be testing what they think they are.

What is the new behavior?

• Adds a couple of sentences to talk about using the existing componentOnReady helper from @ionic/core in Angular's testing.md file
• Adds the same standalone guidance to the testing.md files for React and Vue

Does this introduce a breaking change?

• Yes
• No

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[thetaPC]

Thanks for picking this up, the Angular section is solid. The React and Vue sections are that same section copied across though, and the facts don't carry over (no /standalone route in either test app, no IonicModule outside Angular), so please verify guidance against each framework's own source before duplicating it. Suggestions left inline.

[thetaPC]

Suggested change

	### Testing Standalone Ionic Components
	### Testing Ionic Components

[thetaPC]

Rewritten to describe how components actually load in the React test apps, with the componentOnReady guidance kept as a note for if we add unit tests later.

Suggested change

	Tests for standalone Ionic UI components should only be added under the `/standalone` route. This allows for an isolated environment where the lazy loaded `IonicModule` is not initialized. The standalone components use Stencil's custom element bundle instead of the lazy loaded bundle. If `IonicModule` is initialized then the Stencil components will fall back to using the lazy loaded implementation instead of the custom elements bundle implementation.
	When testing Ionic components, use the exported `componentOnReady` helper from `@ionic/core` instead of calling `el.componentOnReady()` directly. The helper works with both lazy-loaded and custom-element builds, making it more likely the component has finished rendering before making assertions against its rendered DOM or running accessibility tests.
	`@ionic/react` imports every component through `defineCustomElement` from `@ionic/core/components`, so every test runs against the custom elements build.
	These test apps are Cypress only, and Cypress retries assertions until they pass, so there is nothing to wait on manually today. If we add unit tests that assert against rendered DOM, use the `componentOnReady` helper exported from `@ionic/core` rather than calling `el.componentOnReady()` directly. That method does not exist on custom elements, so the direct call throws. The helper waits one animation frame instead, giving the component's inner contents a chance to render.

[thetaPC]

Suggested change

	### Testing Standalone Ionic Components
	### Testing Ionic Components

[thetaPC]

Suggested change

	Tests for standalone Ionic UI components should only be added under the `/standalone` route. This allows for an isolated environment where the lazy loaded `IonicModule` is not initialized. The standalone components use Stencil's custom element bundle instead of the lazy loaded bundle. If `IonicModule` is initialized then the Stencil components will fall back to using the lazy loaded implementation instead of the custom elements bundle implementation.
	`@ionic/vue` imports every component through `defineCustomElement` from `@ionic/core/components`, so every test runs against the custom elements build.
	These test apps are Cypress only, and Cypress retries assertions until they pass, so there is nothing to wait on manually today. If we add unit tests that assert against rendered DOM, use the `componentOnReady` helper exported from `@ionic/core` rather than calling `el.componentOnReady()` directly. That method does not exist on custom elements, so the direct call throws. The helper waits one animation frame instead, giving the component's inner contents a chance to render.