Aggregate e2e test results into a single JUnit report

[timflannagan]

Feature Request

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This is a follow-up for the e2e parallelization work that was done through #2520.

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Now that the overall e2e suite is split across four, parallel instances, the result of running those individual test jobs results in four JUnit reports (on top of any cluster debugging information) being produced, vs. a single JUnit report like the previous behavior. Combining them into a single report will with future work around building CI metrics to help identify flakes, regressions, etc.

[timflannagan]

See #2520 (comment) for more information.