tests: Assume failure in case the async task vanishes.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

web/tests/lib/index.cjs

@@ -101,6 +101,10 @@ async function run_one_module(file) {
 
 test.set_verbose(files.length === 1);
 
+// In case someone mistakenly vanishes the async task with something like `await
+// new Promise(() => {})`, assume failure until we establish otherwise.
+process.exitCode = 1;
+
 (async () => {
     let exit_code = 0;
 
@@ -144,8 +148,7 @@ test.set_verbose(files.length === 1);
         namespace.finish();
     }
 
-    process.exit(exit_code);
+    process.exitCode = exit_code;
 })().catch((error) => /* istanbul ignore next */ {
     console.error(error);
-    process.exit(1);
 });