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.bad.al

@@ -0,0 +1,29 @@
+codeunit 50405 "Test Enqueue Handler Bad"
+{
+    Subtype = Test;
+
+    [Test]
+    [HandlerFunctions('PostMessageHandler')]
+    procedure PostingShowsConfirmationMessage()
+    begin
+        RunPostingThatMessages();
+        // The verdict was delegated to the handler below — a failed
+        // expectation there may never surface as this test's result.
+    end;
+
+    local procedure RunPostingThatMessages()
+    begin
+        Message('Posting completed.');
+    end;
+
+    [MessageHandler]
+    procedure PostMessageHandler(Message: Text)
+    begin
+        // Asserting inside the handler: if this is wrong the failure can be
+        // swallowed by the Message call, leaving the test falsely green.
+        Assert.AreEqual('Posting completed.', Message, 'Unexpected confirmation message.');
+    end;
+
+    var
+        Assert: Codeunit "Library Assert";
+}