Update UI testing instructions

[guarani]

As @staskus pointed out in #22703 (comment), UI tests are run on the Jetpack app.

Given this, it makes sense for the UI testing docs to suggest running Jetpack tests, as this is most likely what most contributors will want to run.

To test: Check if the above makes sense.

Regression Notes

1. Potential unintended areas of impact: Not applicable
2. What I did to test those areas of impact (or what existing automated tests I relied on): Not applicable
3. What automated tests I added (or what prevented me from doing so): Not applicable

PR submission checklist: Not applicable

Testing checklist: Not applicable

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr22719-535dd0a
	Version	24.3
	Bundle ID	org.wordpress.alpha
	Commit	535dd0a
	App Center Build	WPiOS - One-Offs #9010

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22719-535dd0a
	Version	24.3
	Bundle ID	com.jetpack.alpha
	Commit	535dd0a
	App Center Build	jetpack-installable-builds #8047

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[staskus]

Thanks for the documentation update!