Tests: check for DAC override capability instead of using geteuid() #94315

Feature or enhancement

Several tests use os.geteuid() == 0 to check whether the current effective user can override discretionary access control and bypass permission checks for files. While user root typically has CAP_DAC_OVERRIDE, it may not actually have the capability. Non-root accounts can have CAP_DAC_OVERRIDE, too. On wasm32-emscripten it is not possible to query the actual effective uid.

I propose to add a check for DAC override instead of relying on effective uid check.