test: consolidate utm helper tests into one suite

[sadjow]

The tests for src/runtime/utm.ts were split between test/unit.test.ts and test/unit/utm.test.ts. They now live in test/utm.test.ts, next to history.test.ts and storage.test.ts, so the history, storage, and utm helpers each have one test file named after them.

The suite keeps all 22 cases. The 13 getGCLID cases become one table, and the shared fixture is a typed DataObject instead of a JSON string. readLocalData now receives its storage through the existing parameter rather than a mock assigned to global.localStorage, so the suite no longer depends on, or replaces, a global that newer Node versions define.

This prepares the file for the upcoming change that adds more campaign parameters, which extends these helpers.

Validation: pnpm lint --max-warnings 0, pnpm test:types, Prettier on the new file, and pnpm test (81 tests) pass.

[sadjow]

Review notes before merging:

• All 22 cases from test/unit.test.ts and test/unit/utm.test.ts are still here with the same inputs and expectations, and the two urlHasUtmParams checks got stricter (toBe(true) and toBe(false) instead of truthy and falsy). Nothing under src/ changed.
• The description said every runtime module has a test file named after it, which isn't true for composables.ts or plugin.ts. I corrected it to name the helpers it covers.
• The entry fixture and the history ref are now shared across tests. Nothing mutates them, so it's safe today, but new tests here should keep deriving objects with spreads instead of changing these in place.