Add comprehensive test suite, fix minor issues, rename function and file names to match approved verbs (#708)

Tests/Get-TargetUserForAppRemoval.Tests.ps1

@@ -0,0 +1,13 @@
+BeforeAll {
+    . (Join-Path $PSScriptRoot '..\Scripts\Helpers\Get-TargetUserForAppRemoval.ps1')
+}
+
+Describe 'Get-TargetUserForAppRemoval' {
+    It '<Case>' -ForEach @(
+        @{ Case = 'defaults to all users'; Params = @{}; Expected = 'AllUsers' }
+        @{ Case = 'returns an explicit target unchanged'; Params = @{ AppRemovalTarget = 'Alice' }; Expected = 'Alice' }
+    ) {
+        $script:Params = $Params
+        Get-TargetUserForAppRemoval | Should -Be $Expected
+    }
+}