Ensure watcher tests exit cleanly

Always await the last pending state when the watch runs have completed.

Use a teardown hook to ensure the watcher is aborted and has exited before ending the test.

Ensure the item is always an object, even if it didn't come from the generator.

test/watch-mode/helpers/watch.js

@@ -94,22 +94,40 @@ export const withFixture = fixture => async (t, task) => {
 								t.fail('Watcher performed a test run while it should have been idle');
 							}
 						}
-
-						return {};
-					});
+					}).then(() => ({}));
 					idlePromise = promise;
 
 					await promise;
 				};
 
 				let state = {};
 				let pendingState;
+				let process;
+
+				t.teardown(async () => {
+					if (process?.connected) {
+						process.send('abort-watcher');
+					}
+
+					// Sending the `abort-watcher` message should suffice, but on Linux
+					// the recursive watch handle does not close properly. See
+					// <https://github.com/nodejs/node/issues/48437> but there seem to be
+					// other isues.
+					setTimeout(() => {
+						process.kill('SIGKILL');
+					}, 1000).unref();
+
+					try {
+						await process;
+					} catch {}
+				});
 
 				const results = run(args, options);
 				try {
 					let nextResult = results.next();
-					while (true) { // eslint-disable-line no-constant-condition
+					while (!isDone) { // eslint-disable-line no-unmodified-loop-condition
 						const item = await Promise.race([nextResult, idlePromise, donePromise]); // eslint-disable-line no-await-in-loop
+						process ??= item.value?.process;
 
 						if (item.value) {
 							failedIdleAssertion ||= assertingIdle;
@@ -124,8 +142,8 @@ export const withFixture = fixture => async (t, task) => {
 							}
 						}
 
-						if (item.done || isDone) {
-							item.value?.process.send('abort-watcher');
+						if (item.done) {
+							await pendingState; // eslint-disable-line no-await-in-loop
 							break;
 						}
 					}