Add historical views for individual tests

## Context

A recurring workflow from user feedback is starting from a particular test and asking how it has behaved over time. For example, a user investigating rt-tests/cyclictest wanted to see its recent results without navigating through devices individually.

The dashboard currently makes it possible to inspect results from a particular checkout, but it is difficult to answer: “How has this test behaved in recent KernelCI runs?”

## Goal

Provide a way to view the history of a specific test across KernelCI runs. The view should help users identify changes in behavior and investigate when a failure or regression started.

Points to discuss
• What should identify a test across different runs.
• Whether history should span multiple trees or be scoped to a tree/branch by default.
• Filtering by hardware, architecture, configuration, and other dimensions.
• How flaky results should be represented.
• Whether this should be part of a dedicated test-centric page.
• How much history should be exposed and how it should be presented.

Start with the existing dashboard flow for inspecting results from a particular checkout, as described in the issue. Clarify test identity, tree and branch scope, filtering dimensions, flaky-result handling, history limits, and presentation; done means these decisions become acceptance criteria for the historical test view.

Written by the indexing model from the issue text.