Add 3 testing knowledge files from book review

- test-one-when-per-test: one WHEN per test, split if multiple actions
- ui-test-codeunit-naming: _UT suffix for TestPage-based codeunits
- test-feature-scenario-tags: [FEATURE]/[SCENARIO] comment structure

Based on patterns from Automatiserede tests med Business Central (Dieringer).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

custom/knowledge/testing/test-one-when-per-test.md

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+---
+bc-version: [all]
+domain: testing
+keywords: [test, when, scenario, single-action, bdd, atdd, given-when-then]
+technologies: [al]
+countries: [w1]
+application-area: [all]
+---
+
+## Description
+
+Each test procedure must contain exactly **one** `[WHEN]` block — one action that
+triggers the behaviour under test. A test with multiple WHENs ("do A, then do B,
+then check C") is really two or more tests in disguise. Split them.
+
+This constraint serves two purposes:
+
+1. **Failure isolation** — when the test fails you know which action caused it.
+2. **Readable specification** — each test reads as a single, falsifiable claim
+   about the system's behaviour.
+
+A scenario that genuinely requires a precondition action (e.g. "post an order so
+that a ledger entry exists") belongs in `[GIVEN]`. Only the action being asserted
+belongs in `[WHEN]`.
+
+## Anti Pattern
+
+```al
+// WRONG: two actions in one test
+[Test]
+procedure GetPrice_ThenGetDiscount_ReturnsCorrectValues()
+var
+    UnitPrice, LineDiscPct: Decimal;
+begin
+    // [GIVEN] ...
+    // [WHEN] first action
+    FindPriceMgt.GetSalesPrice(CustomerNo, ItemNo, '', UnitPrice, LineDiscPct);
+    // [WHEN] second action — this is a second test in disguise
+    FindPriceMgt.GetSalesPriceTiers(CustomerNo, ItemNo, '', TempBuffer);
+    // [THEN] asserting two unrelated things
+    Assert.AreEqual(100, UnitPrice, '');
+    Assert.IsFalse(TempBuffer.IsEmpty(), '');
+end;
+```
+
+## Best Practice
+
+```al
+// CORRECT: split into two focused tests
+
+[Test]
+procedure GetPrice_CustomerPrice_ReturnsCorrectUnitPrice()
+var
+    UnitPrice, LineDiscPct: Decimal;
+begin
+    // [GIVEN] a customer with a price list line at 100
+    WarecoLib.GivenCustomerWithPrice(Customer, Item, '', 100);
+    // [WHEN]
+    FindPriceMgt.GetSalesPrice(Customer."No.", Item."No.", '', UnitPrice, LineDiscPct);
+    // [THEN]
+    Assert.AreEqual(100, UnitPrice, 'Unit price must match price list');
+end;
+
+[Test]
+procedure GetPriceTiers_CustomerTier_ReturnsOneTierLine()
+var
+    TempBuffer: Record "Find Price Tier Buffer" temporary;
+begin
+    // [GIVEN] a customer with a tier price at min qty 10
+    WarecoLib.GivenCustomerWithTierPrice(Customer, Item, '', 10, 90);
+    // [WHEN]
+    FindPriceMgt.GetSalesPriceTiers(Customer."No.", Item."No.", '', TempBuffer);
+    // [THEN]
+    Assert.AreEqual(1, TempBuffer.Count(), 'Exactly one tier line expected');
+end;
+```
+
+## Naming implication
+
+The procedure name should make the single WHEN self-evident.
+A name with "And" or "Then" in the middle is a strong signal to split:
+
+- `GetPrice_AndDiscount_ReturnsValues` → split
+- `GetPrice_CustomerPrice_ReturnsUnitPrice` → good