Use native Deno testing instead of Vitest

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Vitest is slow with Deno. While @deno/vite-plugin may become faster with time (denoland/deno-vite-plugin#55), there is seemingly a low cost of just shifting to @std/testing and @std/expect.

Issues:

• Absence of inline snapshots
• onTestFinished() hook in Vitest is useful for usePeer() helper. Maybe Deno test context can provide similar functionality.

Steps:

• test: use Deno for unit tests #244
• Move integration tests to Deno
• Write some integration tests with actual Node & Vitest