Draft: Show downstream tests in pipeline test summary

What does this MR do and why?

For Pipeline test page does not include test report... (#363019)

On the Pipeline page in the Tests tab, this MR gets the test summaries for the pipeline's downstream pipelines via GraphQL, adds their statistics to the Summary section, adds their suites onto the list of suites, and adds a badge+link to downstream suites that leads to the pipeline page for the downstream pipeline.

⚠ Caveats with the way this is written so far:

• This may negatively impact the performance of an already-known-to-be-slow page
• This gets only the first 20 downstream pipelines, and only the first 20 test suites from each of them
• I haven't updated any tests
• The count displayed on the Tests tab does not match the summary (downstream tests are included in the summary but not the tab count)
• The issue requested this feature only for child pipelines, but this includes all downstream pipelines (I haven't found a way to distinguish between child pipelines and multi-project pipelines with what we can get from GraphQL)

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