Tests running slow in Firefox and JSDom

### Description
These tests started running slower, throwing timeout exceptions in Firefox and JSDom suites.

- [ ] [packages/web-forms/tests/components/OdkWebForm.test.ts](https://github.com/getodk/web-forms/blob/main/packages/web-forms/tests/components/OdkWebForm.test.ts)
- [ ] [packages/scenario/test/form-definition-validity.test.ts](https://github.com/getodk/web-forms/blob/main/packages/scenario/test/form-definition-validity.test.ts)
- [ ] [packages/scenario/test/smoketests/child-vaccination.test.ts](https://github.com/getodk/web-forms/blob/main/packages/scenario/test/smoketests/child-vaccination.test.ts)
- [ ] [packages/scenario/test/smoketests/who-va.test.ts](https://github.com/getodk/web-forms/blob/main/packages/scenario/test/smoketests/who-va.test.ts)

The issue started after upgrading to the latest versions of Vitest, Vite, and JSDom. These tests now have bigger timeout setting.

Investigate the cause and fix the tests so they don't need a bigger timeout.
- Something related to repeats?
- Something related to the reactive state (SolidJS)?
- ___?

Split the ticket if necessary.

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### Related Issues

- https://github.com/getodk/web-forms/pull/360

Start with packages/web-forms/tests/components/OdkWebForm.test.ts and the three files under packages/scenario/test listed in the issue, comparing their Firefox and JSDom timeout behavior after the Vitest, Vite, and JSDom upgrades. Investigate whether repeats or SolidJS reactive state contribute; done means the affected suites pass without larger timeout settings.

Written by the indexing model from the issue text.