Increase maximum runspace pool size in flaky test

[bergmeister]

PR Summary

Increase maximum number over time until we find a stable upper limit. It turns out one cannot do a straightforward calculation of runspaces (either due to complexity or garbage collection)
An example build failure with 14 runspaces is here: https://ci.appveyor.com/project/PowerShell/psscriptanalyzer/builds/28399656/job/8d7spi13o6kekjaw?fullLog=true

cc @thomasrayner for visiblity

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[bergmeister]

re-trigger ci