Fix flaky test

[crawlingcub]

The test testSamplePairWithUnevenProbabilities always fails if the seed setting line is removed. A better way to test might be to check if the sampled value exists in the set of values that were put in the tree. This eliminates the dependence on seeds -- which may cause problems later.

Please let me know if this looks reasonable. I will be happy to incorporate any other suggestions that you may have. Thanks!

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[crawlingcub]

@googlebot I signed it!

[crawlingcub]

Hi, I fetched and updated the branch. Can you merge into master?

[psc-g]

hi, apologies for not getting back to you earlier.

at this point we're not taking external contributions (mostly for technical reasons). however, we're looking into changing this policy.

regarding setting the seed, i think your suggestion seems reasonable. if you don't mind, let's keep this issue open and we can come back to it once we've figured out the new policy for accepting external contributions.

thanks!