test: prevent svelte css hashes changing based on absolute path

previous behavior broke snapshot tests in CI. caused by sveltejs/svelte#16740

Author: sfriedel

testsuite/tests/dom/__snapshots__/svelte@5/examples.test.ts.snap

@@ -836,7 +836,7 @@ exports[`Example tutorial-localize-script/App.svelte > should render 1`] = `
     </button>
      
     <div
-      class="toast-container svelte-xtg016"
+      class="toast-container svelte-166jqou"
     >
       
     </div>

testsuite/tests/ssr/__snapshots__/svelte@5/examples.test.ts.snap

@@ -178,8 +178,8 @@ exports[`Example tutorial-localize-derived/App.svelte > should render 1`] = `
 
 exports[`Example tutorial-localize-script/App.svelte > should render 1`] = `
 {
-  "body": "<!--[--><div class="space-y-2"><button>Add to Cart</button> <div class="toast-container svelte-xtg016"><!--[--><!--]--></div><!----><!----></div><!--]-->",
+  "body": "<!--[--><div class="space-y-2"><button>Add to Cart</button> <div class="toast-container svelte-166jqou"><!--[--><!--]--></div><!----><!----></div><!--]-->",
   "head": "",
-  "html": "<!--[--><div class="space-y-2"><button>Add to Cart</button> <div class="toast-container svelte-xtg016"><!--[--><!--]--></div><!----><!----></div><!--]-->",
+  "html": "<!--[--><div class="space-y-2"><button>Add to Cart</button> <div class="toast-container svelte-166jqou"><!--[--><!--]--></div><!----><!----></div><!--]-->",
 }
 `;

testsuite/vitest.config.js

@@ -39,7 +39,11 @@ const testConfigSsr = {
 export default defineConfig({
 	plugins: [
 		svelteFluent(),
-		svelte(),
+		svelte({
+			compilerOptions: {
+				cssHash: ({ hash, css }) => `svelte-${hash(css)}`
+			}
+		}),
 		Icons({ compiler: 'svelte', autoInstall: true }),
 		isSsrTest ? undefined : svelteTesting()
 	].filter(Boolean),