Use 'RequireAdminOnWindows' tag in Set-Date tests (#6034)

* Use 'RequireAdminOnWindows' tag instead of 'Test-IsElevated' function.

* Fix incorrect condition of user privilege in Linux/macOS CI

test/powershell/Modules/Microsoft.PowerShell.Utility/Set-Date.Tests.ps1

@@ -1,22 +1,20 @@
-# first check to see which platform we're on. If we're on windows we should be able
-# to be sure whether we're running elevated. If we're on Linux, we can use whoami to
-# determine whether we're elevated
-Describe "Set-Date" -Tag "CI" {
-    BeforeAll {
-        $IsElevated = Test-IsElevated
-    }
-
-    It "Set-Date should be able to set the date in an elevated context" -Skip:(! $IsElevated) {
+Describe "Set-Date for admin" -Tag @('CI', 'RequireAdminOnWindows') {
+    # Currently, CI tests on Linux/macOS are always run as normal user. So we need to skip these tests on non-Windows platform.
+    # CI tests in root privilege on Linux/macOS is not supported.
+    # See : https://github.com/PowerShell/PowerShell/issues/5645
+    It "Set-Date should be able to set the date in an elevated context" -Skip:(!$IsWindows) {
         { Get-Date | Set-Date } | Should Not Throw
     }
 
-    It "Set-Date should be able to set the date with -Date parameter" -Skip:(! $IsElevated) {
+    It "Set-Date should be able to set the date with -Date parameter" -Skip:(!$IsWindows) {
         $target = Get-Date
         $expected = $target
         Set-Date -Date $target | Should Be $expected
     }
+}
 
-    It "Set-Date should produce an error in a non-elevated context" -Skip:($IsElevated) {
+Describe "Set-Date" -Tag 'CI' {
+    It "Set-Date should produce an error in a non-elevated context" {
         { Get-Date | Set-Date } | ShouldBeErrorId "System.ComponentModel.Win32Exception,Microsoft.PowerShell.Commands.SetDateCommand"
     }
 }