Increase shard count in selected regression tests to reflect real-world usage

(edited)

We used to run most of our regression tests with the default shard count. We recently reduced the shard count number from 32 to 4. This change speeded up our regression tests, but might have reduced coverage for certain code paths.

@furkansahin and @ozgune chatted yesterday. We decided to make two changes to mitigate this:

1. Add (or change) a regression to test a scenario where Citus has ~100 shards. This is to mitigate the issue [@mtuncer found](https://github.com/citusdata/citus/issues/1828#issuecomment-353928507) in Windows, where we can't have reliably open more than 64 connections.
2. Change 3-4 regression tests and increase these tests' shard count back to 32. Customers start using Citus today with a shard count of 32. We'd like our regression tests to cover more of the default customer experience scenario. If we have regression tests that reflect the tutorials in our website, those could be good candidates for increased shard count.