test: extract setUp in BT1226IfNotNilMutationsTest

[jamesc]

Summary

• All 6 test methods in BT1226IfNotNilMutationsTest opened with an identical a := IfNotNilMutActor spawn line, duplicating the fixture setup in every test.
• Extract to a setUp method so the shared fixture is declared once. If the fixture class ever changes, there is one place to update instead of six.
• Replace local variable a with self.a throughout. No assertion logic changed.

Test plan

• just test-bunit passes: 250 files, 2666 tests, 0 failed (verified locally before and after the change).
• Test count is unchanged — no tests were removed or added.

Smell

rg --count '// => _' stdlib/test/ and manual inspection surfaced this file as a repeated-setup candidate: six identical spawn lines across six test methods, no setUp, no tearDown needed.

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Generated by Claude Code

[github-actions]

✅ Claude Review — PASS

Code Review Summary

Overall: PR is clean — no blockers, no suggestions, no nits.

Changes Reviewed

The PR refactors bt_1226_if_not_nil_mutations_test.bt in two commits:

1. Extract setUp (6eb9a587): Removes the repeated a := IfNotNilMutActor spawn at the top of each of the six test methods, replaces with a shared field: a = nil, setUp => self.a := IfNotNilMutActor spawn, and rewrites all a … references to self.a ….

2. Add tearDown (1f7eda0e, this push): Adds tearDown => self.a stop so the actor is cleaned up after each test.

Correctness Assessment

• Each test receives a fresh actor: setUp is called per-test in BUnit, so self.a is always a newly spawned IfNotNilMutActor with empty running state. No cross-test state bleed.
• self.a := IfNotNilMutActor spawn in setUp is the same pattern used by ConditionalHofStateTest and actor_threading_constructs_test; the return value correctly threads the updated TestCase instance to each test method.
• tearDown => self.a stop matches the bare-stop pattern used by ConditionalHofStateTest, actor_threading_constructs_test, and actor_self_send_collect_test. stop is documented as idempotent. IfNotNilMutActor has no custom initialize, so spawn cannot fail and the nil case is unreachable.
• No tests stop the actor mid-method, so there is no double-stop scenario.
• The test body logic is unchanged; only the actor lifecycle scaffolding is refactored.