Call res.end() when snapshot file is missing

This accidentally made this request hang forever if the snapshot file
does not exist.

Bug: 434252192
Change-Id: I402043c95ba4331a3c349e7d94ade0ef0339f247
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6961120
Reviewed-by: Paul Irish <[email protected]>
Auto-Submit: Connor Clark <[email protected]>
Commit-Queue: Paul Irish <[email protected]>

Author: Connor Clark

test/unit/karma.conf.ts

@@ -291,6 +291,7 @@ function snapshotTesterFactory() {
       const snapshotPath = path.join(SOURCE_ROOT, parsedUrl.query.snapshotPath);
       if (!fs.existsSync(snapshotPath)) {
         res.writeHead(404);
+        res.end();
         return;
       }