build: when running test, wait until emulator starts properly

script/test

@@ -12,6 +12,7 @@ if [ "$EMULATOR" = 1 ]; then
     firmware/trezor.elf &
     sleep 1
     export TREZOR_PATH=udp:127.0.0.1:21324
+    "${PYTHON:-python}" script/wait_for_emulator.py
 fi
 
 export TREZOR_TRANSPORT_V1=1

script/wait_for_emulator.py

@@ -0,0 +1,31 @@
+#!/usr/bin/env python3
+
+import socket
+import sys
+import time
+
+DEFAULT_ADDR = "127.0.0.1:21324"
+
+if len(sys.argv) > 1:
+    addr = sys.argv[1]
+else:
+    addr = DEFAULT_ADDR
+
+host, port = addr.split(":")
+SOCK_ADDR = (host, int(port))
+
+sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+sock.connect(SOCK_ADDR)
+sock.settimeout(0)
+
+start = time.monotonic()
+while True:
+    try:
+        sock.sendall(b"PINGPING")
+        r = sock.recv(8)
+        if r == b"PONGPONG":
+            break
+    except Exception:
+        time.sleep(0.05)
+end = time.monotonic()
+print("waited for {:.3f}s".format(end - start))