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.bad.al

@@ -0,0 +1,32 @@
+codeunit 50403 "Test Table Relation Bad"
+{
+    Subtype = Test;
+
+    [Test]
+    procedure SalesLineAcceptsExistingItem()
+    var
+        Customer: Record Customer;
+        SalesHeader: Record "Sales Header";
+        SalesLine: Record "Sales Line";
+    begin
+        LibrarySales.CreateCustomer(Customer);
+        LibrarySales.CreateSalesHeader(SalesHeader, SalesHeader."Document Type"::Order, Customer."No.");
+
+        // No item was created. Validating "No." against a non-existent item
+        // raises a TableRelation error here, aborting the test at runtime
+        // before any assertion runs.
+        SalesLine.Init();
+        SalesLine."Document Type" := SalesHeader."Document Type";
+        SalesLine."Document No." := SalesHeader."No.";
+        SalesLine."Line No." := 10000;
+        SalesLine.Type := SalesLine.Type::Item;
+        SalesLine.Validate("No.", 'GHOST');
+        SalesLine.Insert(true);
+
+        Assert.AreEqual('GHOST', SalesLine."No.", 'Unreachable: validation already failed.');
+    end;
+
+    var
+        Assert: Codeunit "Library Assert";
+        LibrarySales: Codeunit "Library - Sales";
+}