Add testing knowledge: UI handlers, table relations, asserterror, fixtures (P1+P2)

Six BC-specific testing-domain knowledge articles in community/knowledge/testing/, each with .good.al/.bad.al samples:

- ui-calls-require-test-handlers
- tablerelation-requires-prerequisite-records
- handlers-enqueue-never-assert
- handlerfunctions-attribute-must-match-ui-path
- asserterror-needs-expectederror-and-code
- use-library-codeunits-for-test-fixtures

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

community/knowledge/testing/handlers-enqueue-never-assert.good.al

@@ -0,0 +1,36 @@
+codeunit 50404 "Test Enqueue Handler Good"
+{
+    Subtype = Test;
+
+    [Test]
+    [HandlerFunctions('PostMessageHandler')]
+    procedure PostingShowsConfirmationMessage()
+    var
+        ActualMessage: Text;
+    begin
+        // [WHEN] the code under test posts and raises a Message
+        RunPostingThatMessages();
+
+        // [THEN] the body — not the handler — owns the verdict
+        ActualMessage := LibraryVariableStorage.DequeueText();
+        Assert.AreEqual('Posting completed.', ActualMessage, 'Unexpected confirmation message.');
+        LibraryVariableStorage.AssertEmpty();
+    end;
+
+    local procedure RunPostingThatMessages()
+    begin
+        // Stands in for the production routine that ends with a Message.
+        Message('Posting completed.');
+    end;
+
+    [MessageHandler]
+    procedure PostMessageHandler(Message: Text)
+    begin
+        // Capture only — never assert here.
+        LibraryVariableStorage.Enqueue(Message);
+    end;
+
+    var
+        Assert: Codeunit "Library Assert";
+        LibraryVariableStorage: Codeunit "Library - Variable Storage";
+}