chore: add support for HMR env flag (#12420)

* chore: add support for HMR env flag

* chore: add support for HMR env flag

* chore: add support for HMR env flag

* chore: add support for HMR env flag

Author: trueadm

packages/svelte/tests/runtime-legacy/shared.ts

@@ -141,6 +141,8 @@ async function common_setup(cwd: string, runes: boolean | undefined, config: Run
 	const compileOptions: CompileOptions = {
 		generate: 'client',
 		rootDir: cwd,
+		dev: process.env.HMR ? true : undefined,
+		hmr: process.env.HMR ? true : undefined,
 		...config.compileOptions,
 		immutable: config.immutable,
 		accessors: 'accessors' in config ? config.accessors : true,