test: add session with prerelease dependencies

[tswast]

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Towards #426 🦕

[tswast]

Internal configs added in change 412984922. Once submitted, I can trigger the Kokoro prerelease job. Currently expected to fail. Won't block merges.

[plamut]

Looks good, but I'll wait until the external CI issue is resolved to see if OwlBot does not override anything.

[tseaver]

@tswast

Once submitted, I can trigger the Kokoro prerelease job. Currently expected to fail. Won't block merges.

Do we even want to run the prerelease checks during a presubmit? ISTM that they are better caught in nightlies.

[tswast]

Do we even want to run the prerelease checks during a presubmit? ISTM that they are better caught in nightlies.

I agree that nightlies are the most useful (assuming flakeybot actually looks at these jobs. I think it should), though for issues like #426 where we know we want to get ahead of some breaking changes, it's nice to have it as a presubmit.

[plamut]

Let's see if Owlbot has anything to say now with updated config...