cleanup unawaited futures lints (#5945)

dev/devicelab/lib/framework/framework.dart

@@ -83,7 +83,7 @@ class _TaskRunner {
     } on TimeoutException catch (_) {
       return new TaskResult.failure('Task timed out after $taskTimeout');
     } finally {
-      forceQuitRunningProcesses();
+      await forceQuitRunningProcesses();
       _closeKeepAlivePort();
     }
   }

dev/devicelab/lib/framework/runner.dart

@@ -34,6 +34,7 @@ Future<Map<String, dynamic>> runTask(String taskName) async {
 
   bool runnerFinished = false;
 
+  // ignore: unawaited_futures
   runner.exitCode.then((_) {
     runnerFinished = true;
   });

dev/devicelab/lib/framework/utils.dart

@@ -166,6 +166,7 @@ Future<Process> startProcess(String executable, List<String> arguments,
   ProcessInfo procInfo = new ProcessInfo(command, proc);
   _runningProcesses.add(procInfo);
 
+  // ignore: unawaited_futures
   proc.exitCode.then((_) {
     _runningProcesses.remove(procInfo);
   });