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/asserterror-needs-expectederror-and-code.good.al

@@ -0,0 +1,23 @@
+codeunit 50408 "Test AssertError Good"
+{
+    Subtype = Test;
+
+    [Test]
+    procedure BlankNameIsRejectedWithSpecificError()
+    var
+        Customer: Record Customer;
+    begin
+        Customer.Init();
+        Customer.Name := '';
+
+        // [WHEN] a mandatory field is blank
+        asserterror Customer.TestField(Name);
+
+        // [THEN] verify the SPECIFIC failure — message and code — not just "any error"
+        Assert.ExpectedError('Name must have a value');
+        Assert.ExpectedErrorCode('TestField');
+    end;
+
+    var
+        Assert: Codeunit "Library Assert";
+}