Rollup merge of #111936 - ferrocene:pa-test-suite-metadata, r=jyn514

Include test suite metadata in the build metrics

This PR enhances the build metadata to include structured information about the test suites being executed, allowing external tools consuming the metadata to understand what was being tested.

The included metadata is:

* Target triple
* Host triple
* Stage number
* For compiletest tests:
  * Suite name
  * Mode
  * Comparing mode
* For crate tests:
  * List of crate names

This is implemented by replacing the `test` JSON node with a new `test_suite` node, which contains the metadata and the list of tests. This change also improves the handling of multiple test suites executed in the same step (for example in compiletest tests with a compare mode), as the multiple test suite executions will now be tracked in separate `test_suite` nodes.

This included a breaking change in the build metrics metadata format. To better handle this, in the second commit this PR introduces the `metadata_version` top-level field. The old version is considered to be `0`, while the new one `1`. Bootstrap will also gracefully handle existing metadata of a different version.

r? `@jyn514`

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