Bail out of create process if it's been disposed

This is likely the cause of several terminal test issues we've been seeing,
especially on remote. When process creation becomes async, the process
seems to hang around if the terminal was disposed during the process
creation.

Another thing to consider if env var relaunches messing with tests.

extensions/vscode-api-tests/src/singlefolder-tests/workspace.tasks.test.ts

@@ -103,6 +103,11 @@ import { assertNoRpc } from '../utils';
 				}));
 				terminal.dispose();
 			});
+
+			// Wait for end of task process
+			// await new Promise<void>(r => {
+			// 	disposables.push(tasks.onDidEndTaskProcess(() => r));
+			// });
 		});
 
 		test('sync CustomExecution task should flush all data on close', async () => {