Consolidate approach for conditional tests that check for privileged processes (...

Consolidate approach for conditional tests that check for privileged processes (#78793)

* Consolidate approach for conditional tests that check for privileged processes

* Simpler privileged process helpers

* Revert SLN file changes

* Remove platform-specific helpers for privileged process conditions

* Fix System.IO tests for unix-specific privileged process scenarios

* Browser does not support privileged processes

* Fix System.IO tests for unix-specific privileged process scenarios

* Enable the Environment.IsPrivilegedProcess test for browser

* PR feedback

Omit [PlatformSpecific] in modified tests that are in platform-specific test files.

Remove [Outerloop] attributes from privileged process tests.

Better multithreaded handling in PlatformDetection.IsPrivilegedProcess