Add ability to view multiple test groups in the same tab.

[michelle192837]

Some tabs compile many result sources together (for instance, a tab tracking the overall health of all 100 Prow jobs owned by the same team). This has a few problems:

• It's inefficient to update (we need to fetch the results for each result source each update), and may not be able to update within the instance-wide timeout we have set for test groups.
• It's also sometimes not correct; incremental updates may miss old results, especially if jobs finish at completely different times or have very different durations.
• It may be redundant with info we already have in TestGrid. This team that owns 100 Prow jobs may have a reasonable setup where they have 1 tab for each job individually (to track the health of the tests run by each job in detail), and another overview tab that has the high-level status for all 100 jobs. Theoretically we should be able to compile an overview tab for those jobs, from the data we already have on each individual job, but that's not how configuration works right now.

We can help support this case! At the moment, a dashboard tab is backed by 1 test group, so a tab that wants to show 100 Prow jobs must set up all 100 Prow jobs as the test group source. But if we can get a tab by pulling the results from multiple test groups, instead we can set up 1 test group for each Prow job, and back 1 tab with those 100 test groups.

Specifics

• Update the Tabulator to correctly combine results from multiple test groups into a single tab.
• Update configuration to let multiple test groups back a single tab.
  • Currently each tab is backed by exactly 1 test group

Contribution
If you're interested in working on this, let us know and we're happy to discuss more! This should have a design before coding starts (and we can help out), but it should be doable for someone with a bit of experience with TestGrid's architecture, specifically how the Updater and Tabulator work.