Improve test discovery and introduce test-centric navigation

## Context

Feedback from KernelCI users has highlighted that finding a specific test or group of tests in the dashboard is not straightforward.

For example, a maintainer interested in a specific selftest should be able to answer questions such as:
• Where can I find this test in KernelCI?
• On which trees, architectures, hardware, and configurations is it running?
• What are its latest results?
• How can I follow a collection of related tests?

The current dashboard navigation is primarily organized around tree checkouts. This works well when starting from a particular kernel revision, but maintainers may instead start from the tests or subsystems they care about.

## Goal

Explore and implement a more test-centric way of discovering and navigating KernelCI results. A user should be able to start from a test name, path, suite, or collection and discover the relevant results without first knowing which tree, checkout, or hardware contains them.

Points to discuss
• Global search for tests across trees and hardware.
• Whether tests should have their own dedicated view/page.
• Searching by test name, path, suite, or other metadata.
• Support for user-defined or predefined collections of tests.
• How this should interact with the existing tree-centric navigation.
• Which filters should be available after selecting a test.

Start by reviewing the existing tree-centric dashboard navigation and how users currently locate results. Define a test-centric flow covering search by test name, path, suite, or metadata, result filters, and test collections; done means users can discover relevant results without first selecting a tree, checkout, or hardware.

Written by the indexing model from the issue text.