Refactor scrollable_semantics_test.dart to avoid shared top-level SemanticsTester #189799

What needs cleaning up

packages/flutter/test/widgets/scrollable_semantics_test.dart keeps a single SemanticsTester in a top-level variable that is reused across every test in the file:

void main() {
  late SemanticsTester semantics;
  ...
}

Sharing one tester across tests is fragile. If a test doesn't dispose of it (for example, when a test fails partway through), that state can leak into the next test and produce confusing, order-dependent failures.

What to do

Refactor the file so each test owns its own SemanticsTester and disposes of it reliably. A setUp/tearDown pair, or addTearDown inside each test, both work well here. After the change there should be no shared top-level SemanticsTester.

Scope

• Only packages/flutter/test/widgets/scrollable_semantics_test.dart.
• Test-only change; no framework behavior changes.
• All existing tests in the file should still pass.

Context

Noticed during review of #188788. It was intentionally left out of that PR to keep it focused on the fix, and tracked here for a separate follow-up.