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

Scripts/Helpers/Get-TargetUserForAppRemoval.ps1

@@ -0,0 +1,9 @@
+# Target is determined from $script:Params["AppRemovalTarget"] or defaults to "AllUsers"
+# Target values: "AllUsers" (removes for all users + from image), "CurrentUser", or a specific username
+function Get-TargetUserForAppRemoval {
+    if ($script:Params.ContainsKey("AppRemovalTarget")) {
+        return $script:Params["AppRemovalTarget"]
+    }
+    
+    return "AllUsers"
+}