Summary
Enable PPC testing that was disabled with WT-9039. Also investigate if we can increase the stress test coverage on that platform

Motivation
The PPC builds were switched to using RHEL. This caused some failures (see WT-8981) that forced us to temporarily disable the PPC tests altogether (WT-9039).
Power PC as an architecture follows weak memory ordering and has historically helped us find memory synchronization bugs. As we put more effort into strengthening our ARM support, we should not only continue to stress tests on PPC, but if practical increase the stress testing on that platform.

• Does this affect any team outside of WT?
  No

• Is this issue urgent?
  Yes

Acceptance Criteria (Definition of Done)
PPC tests, especially the stress tests are back running. We also had a discussion on increasing the stress test coverage (duration, frequency, etc) and if practical carried on that work too.