[llvm] [ci] Write test results to unique file names (PR #113160)

[David Spickett via llvm-commits]

DavidSpickett wrote:

My plan is to later use https://buildkite.com/docs/agent/v3/cli-annotate#using-annotations-to-report-test-results in the pipeline.

Having the results as artifacts means anyone can use something like https://github.com/lukejpreston/xunit-viewer on their own machine too.

https://github.com/llvm/llvm-project/pull/113160