fix(snapshot): fix concurrent SnapshotClient.startCurrentRun with same filepath

[hi-ogawa]

Description

(EDIT: I made an alternative PR in #4796, which makes more sense to me, but have a little more moving parts.)

Closes #4782
Closes #3361

It seems there is a slight race condition when running multiple SnapshotClient.startCurrrentRun + assert concurrently while each run is synchronous.

Note that this all-synchronous case is not covered by the existing test, such as:

vitest/test/snapshots/test/shapshots-concurrent.test.ts

Lines 10 to 15 in 039814b

	data.forEach((i) => {
	test.concurrent(i, async ({ expect }) => {
	await new Promise(resolve => setTimeout(resolve, Math.random() * 100))
	expect(i).toMatchSnapshot()
	})
	})

The issue is similar to Time-of-check to time-of-use race condition. So, running Map.get/set synchronously ensures SnapshotClient to have unique SnapshotState per filepath.

Though I think this works as a quick fix, the underlying issue might be that VitestTestRunner's current usage of SnapshotClient where SnapshotClient is not designed to run concurrently. I'm exploring a better approach to deal with issue, but if maintainers are happy with current fix, then larger restructuring might not be necessary.

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[hi-ogawa]

This test case is mostly taken from #4783. Thanks @martypdx!