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/tablerelation-requires-prerequisite-records.good.al

@@ -0,0 +1,29 @@
+codeunit 50402 "Test Table Relation Good"
+{
+    Subtype = Test;
+
+    [Test]
+    procedure SalesLineAcceptsExistingItem()
+    var
+        Customer: Record Customer;
+        Item: Record Item;
+        SalesHeader: Record "Sales Header";
+        SalesLine: Record "Sales Line";
+    begin
+        // [GIVEN] the parents exist first: customer, then item
+        LibrarySales.CreateCustomer(Customer);
+        LibraryInventory.CreateItem(Item);
+        LibrarySales.CreateSalesHeader(SalesHeader, SalesHeader."Document Type"::Order, Customer."No.");
+
+        // [WHEN] a dependent sales line references the existing item
+        LibrarySales.CreateSalesLine(SalesLine, SalesHeader, SalesLine.Type::Item, Item."No.", 1);
+
+        // [THEN] the TableRelation on "No." resolves and the line persists
+        Assert.AreEqual(Item."No.", SalesLine."No.", 'Sales line should carry the created item.');
+    end;
+
+    var
+        Assert: Codeunit "Library Assert";
+        LibrarySales: Codeunit "Library - Sales";
+        LibraryInventory: Codeunit "Library - Inventory";
+}