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/use-library-codeunits-for-test-fixtures.bad.al

@@ -0,0 +1,25 @@
+codeunit 50411 "Test Library Fixtures Bad"
+{
+    Subtype = Test;
+
+    [Test]
+    procedure OrderUsesHandRolledFixtures()
+    var
+        Customer: Record Customer;
+        SalesHeader: Record "Sales Header";
+    begin
+        // Hand-rolled customer: a chosen "No." with no number-series entry and
+        // none of the mandatory fields a real customer carries. Bypasses the
+        // setup production code assumes and breaks when the schema adds a
+        // required field this test does not set.
+        Customer.Init();
+        Customer."No." := 'X';
+        Customer.Insert();
+
+        SalesHeader.Init();
+        SalesHeader."Document Type" := SalesHeader."Document Type"::Order;
+        SalesHeader."No." := 'SO-X';
+        SalesHeader.Validate("Sell-to Customer No.", Customer."No.");
+        SalesHeader.Insert(true);
+    end;
+}