perf: pass testfiles at once when --no-isolate --maxWorkers=1

[AriPerkkio]

Description

• Follow-up of TODO comment in feat!: rewrite pools without tinypool #8705
• In Vitest v3 --singleThread/Fork used to pass all testfiles to worker at once
• Also closes Share a single jsdom between suites when running as --no-isolate #8478

Artificial setup with 1000 testfiles which each have 4 empty test cases:

// Before
Test Files  1000 passed (1000)
     Tests  4000 passed (4000)
  Start at  15:31:04
  Duration  5.38s (transform 993ms, setup 0ms, collect 1.85s, tests 76ms, environment 6ms, prepare 51ms)

// After
Test Files  1000 passed (1000)
     Tests  4000 passed (4000)
  Start at  15:31:48
  Duration  4.05s (transform 1.00s, setup 0ms, collect 1.86s, tests 77ms, environment 0ms, prepare 2ms)

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[sheremet-va]

Does this also fix #8478 ?

[AriPerkkio]

Does this also fix #8478 ?

With the example from #8478 (comment) I think it does. On main both of these log false.