Make sure systematic testing doesn't switch to suspended thread

[dipinhora]

Prior to this commit, the systematic testing next thread logic would consider switching to the pinned actor thread even if it was suspended. This was very wasteful since the suspended pinned actor thread would not do any meaningful work and almost immediately re-suspend itself.

This commit changes things so that the next thread logic properly takes into account whether the pinned actor thread is suspended or not when selecting the next thread to activate.

[ponylang-main]

Hi @dipinhora,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 4647.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

@dipinhora can you rebase against main?

[dipinhora]

@dipinhora can you rebase against main?

i didded it