Misc/capture logs when running examples

[zhenyu-ms]

Bug / Requirement Description

certain python package will block signals, which will be inherited by subprocess in App driver, which will later cause SIGTERM failed to be received by subprocess

Solution description

unblock all signals in subprocess.Popen's preexec_fn

Checklist:

• Test
• Example (both test_plan.py and .rst)
• Documentation (API)
• News fragment present for release notes
• MS info leakage check
• For new driver: driver index page
• For new assertion: ui/pdf/std renderers, documentation
• For new cmdline arg: documentation