关于使用subprocess执行ios_minicap时获取不到Banner信息就中断了

执行minicap.py setup方法的时候报错不能显示banner信息
```
#airtest/core/ios/minicap.py

def setup(self):
cmd = [self.executable, "--udid", self.udid, "--port", str(self.port), "--resolution", self.resolution]
proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, stdin=subprocess.PIPE)
nbsp = NonBlockingStreamReader(proc.stdout, print_output=True, name="minicap_sever")
while True:
line = nbsp.readline(timeout=10.0)
if line is None:
raise RuntimeError("minicap setup error")
if b"== Banner ==" in line:
break
if proc.poll() is not None:
logging.warn("Minicap server already started, use old one")
self.server_proc = proc
```
只能获取到这部分的信息：
EnableDALDevices
2019-12-19 15:22:21.821 ios_minicap[62283:3120570] Available devices:
2019-12-19 15:22:21.821 ios_minicap[62283:3120570] 214fce468bacae0998f0d857ea5452548198b1b8
2019-12-19 15:22:21.821 ios_minicap[62283:3120570] DJH434115P8F6VTDV

下面这段信息就没有输出直接报：raise RuntimeError("minicap setup error")
resolution: 320x568
Allocating 555008 bytes for JPEG encoder
== Banner ==
version: 1
size: 24
pid: 62283
real width: 320
real height: 568
desired width: 320
desired height: 568
orientation:
quirks: 1
banner: 1184bf3004010038200401003820001