Spurious Failures on TestPfiClusteringOnDenseFeatures test

[TomFinley]

TestPfiClusteringOnDenseFeatures has, I believe introduced a week ago in PR #1832, has been responsible for some seemingly spurious test failures that are blocking builds. See, e.g., this build here from @Zruty0 for his PR #1920 , where PFI (which is totally unrelated to @Zruty0 's change) is blocking checking it in.

My best guess as to why this is happening is that for this clustering test the number of threads was not set to 1. (That obviously will cause issues, but whether it's the only issue is more than I know right now.) Sometimes you can kinda get away with that (e.g., sometimes results from logistic regression can resemble each other in different configuration settings since it is solving a convex problem), but this is not true of clustering. For that reason, the test TestPfiClusteringOnDenseFeatures has been responsible for multiple spurious test failures since its introduction last week.

We will have a PR to set the threads to 1, at least as a first try to get the situation somewhat under control. Note that this will not be necessary in FastTree since it is engineered in such a way that it gets the same result no matter how many or few threads are used.

/cc @rogancarr @shmoradims @artidoro

[rogancarr]

@TomFinley #1844 removes PFI clustering -- it doesn't really make sense for this learning task.