It works-ish

vite.config.ts

@@ -0,0 +1,33 @@
+import tailwindcss from "@tailwindcss/vite";
+import { svelteTesting } from "@testing-library/svelte/vite";
+import { sveltekit } from "@sveltejs/kit/vite";
+import { defineConfig } from "vite";
+
+export default defineConfig({
+  plugins: [tailwindcss(), sveltekit()],
+  test: {
+    workspace: [
+      {
+        extends: "./vite.config.ts",
+        plugins: [svelteTesting()],
+        test: {
+          name: "client",
+          environment: "jsdom",
+          clearMocks: true,
+          include: ["src/**/*.svelte.{test,spec}.{js,ts}"],
+          exclude: ["src/lib/server/**"],
+          setupFiles: ["./vitest-setup-client.ts"],
+        },
+      },
+      {
+        extends: "./vite.config.ts",
+        test: {
+          name: "server",
+          environment: "node",
+          include: ["src/**/*.{test,spec}.{js,ts}"],
+          exclude: ["src/**/*.svelte.{test,spec}.{js,ts}"],
+        },
+      },
+    ],
+  },
+});