[TUTORIAL]: Using in-memory Postgres when testing with vitest

Sharing for posterity as it may help someone. A `vi.mock` to switch `pg` databases to in-memory `pglite`, push your drizzle schema, and insert seed data. [`pglite`](https://orm.drizzle.team/docs/connect-pglite) runs-WASM compiled postgres in memory, which is ideal for testing as there's no docker containers, no delay, and it's real pg. This solution supports parallelism and watch mode. Most importantly - it uses push in place of migrate, as there's no need to create migration files when testing.

**Note to Drizzle maintainers**, please do not remove `drizzle-kit/api`'s `pushSchema` method. While it's not well documented, it allows us to push schema directly to test dbs. Please don't make me push via child_process execSync again.

Stack:
- Typescript
- Vitest
- Drizzle
- NodePostgres (`drizzle-orm/node-postgres`)

**Mock:**
```typescript
// vitest.setup.ts

vi.mock("src/db", async (importOriginal) => {
const { default: _, ...rest } =
await importOriginal()

const { PGlite } = await vi.importActual<
typeof import("@electric-sql/pglite")
>("@electric-sql/pglite")
const { drizzle } =
await vi.importActual(
"drizzle-orm/pglite",
)

// use require to defeat dynamic require error
// (https://github.com/drizzle-team/drizzle-orm/issues/2853#issuecomment-2668459509)
const { createRequire } =
await vi.importActual("node:module")
const require = createRequire(import.meta.url)
const { pushSchema } =
require("drizzle-kit/api") as typeof import("drizzle-kit/api")

const { schema, User } = rest

const client = new PGlite()
const db = drizzle(client, { schema })

// apply schema to db
const { apply } = await pushSchema(schema, db as any)
await apply()

// seed test data
await db
.insert(User)
.values({ email: "email@email.email", externalId: "external" })

return { default: db, ...rest }
})

```

**Detail:**
```typescript
// vitest.config.mts

import { defineConfig } from "vitest/config"
import tsconfigPaths from "vite-tsconfig-paths"

const config = defineConfig({
plugins: [tsconfigPaths({ root: "./" })],
test: {
globals: true,
environment: "node",
setupFiles: "./vitest.setup.ts",
},
})

export default config
```

```typescript
// src/db/index.ts (mocked, db is not exposed to tests)

import { Pool } from "pg"
import { drizzle } from "drizzle-orm/node-postgres"

import { User } from "./user.sql"

export const schema = {
User,
...
}

const db = drizzle(
new Pool({
...
}),
{
schema,
},
)
export default db

export * from "./user.sql"
```