PR Summary

The event handler test is failing intermittently in our daily test run:
https://powershell.visualstudio.com/PowerShell/_build/results?buildId=35039

The cause of it is likely the SecuritySupportTests, where the registration/unregistration of the event handler are separately in two tests.
I merge those 3 tests into one and will see if that resolves the intermittent failure.

Also slightly update Start-PSBuild to make -SMAOnly literally only re-build and re-deploy S.M.A.dll, without doing extra post-build tasks.
The main purpose of this change is to avoid spinning up the pwsh for retrieving experimental features, so that we can add code in S.M.A.dll to wait for debugger to attach for debugging scenarios. /cc Rob Holt (@rjmholt)

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