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/handlerfunctions-attribute-must-match-ui-path.bad.al

@@ -0,0 +1,32 @@
+codeunit 50407 "Test Handler Match Bad"
+{
+    Subtype = Test;
+
+    // The path raises a Confirm AND a Message, but only the Confirm handler
+    // is listed. The Message has no handler -> unhandled-UI runtime abort.
+    // The mirror mistake — listing a third handler the path never hits —
+    // instead fails with "handler function was not executed".
+    [Test]
+    [HandlerFunctions('ConfirmHandlerYes')]
+    procedure PostWithConfirmAndMessage()
+    begin
+        RunPostingThatConfirmsAndMessages();
+    end;
+
+    local procedure RunPostingThatConfirmsAndMessages()
+    begin
+        if Confirm('Post this document?', false) then
+            Message('Posting completed.');
+    end;
+
+    [ConfirmHandler]
+    procedure ConfirmHandlerYes(Question: Text; var Reply: Boolean)
+    begin
+        Reply := true;
+    end;
+
+    [MessageHandler]
+    procedure PostMessageHandler(Message: Text)
+    begin
+    end;
+}