cli: "./run_electrum daemon -d" to block until daemon becomes ready

Without this,
`$ python3 -m unittest electrum.tests.regtest.TestUnixSockets.test_unixsockets`
was failing on my machine but succeeding on CI, due to timing differences.

electrum/daemon.py

@@ -151,6 +151,19 @@ def request(config: SimpleConfig, endpoint, args=(), timeout: Union[float, int]
         time.sleep(1.0)
 
 
+def wait_until_daemon_becomes_ready(*, config: SimpleConfig, timeout=5) -> bool:
+    t0 = time.monotonic()
+    while True:
+        if time.monotonic() > t0 + timeout:
+            return False  # timeout
+        try:
+            request(config, 'ping')
+            return True  # success
+        except DaemonNotRunning:
+            time.sleep(0.05)
+            continue
+
+
 def get_rpc_credentials(config: SimpleConfig) -> Tuple[str, str]:
     rpc_user = config.RPC_USERNAME or None
     rpc_password = config.RPC_PASSWORD or None

run_electrum

@@ -284,6 +284,7 @@ def sys_exit(i):
 
 
 def main():
+    global loop, stop_loop, loop_thread
     # The hook will only be used in the Qt GUI right now
     util.setup_thread_excepthook()
     # on macOS, delete Process Serial Number arg generated for apps launched in Finder
@@ -416,7 +417,13 @@ def main():
             sys.exit(1)
         if pid:
             print_stderr("starting daemon (PID %d)" % pid)
-            sys.exit(0)
+            loop, stop_loop, loop_thread = create_and_start_event_loop()
+            ready = daemon.wait_until_daemon_becomes_ready(config=config, timeout=5)
+            if ready:
+                sys_exit(0)
+            else:
+                print_stderr("timed out waiting for daemon to get ready")
+                sys_exit(1)
         else:
             # redirect standard file descriptors
             sys.stdout.flush()
@@ -428,7 +435,6 @@ def main():
             os.dup2(so.fileno(), sys.stdout.fileno())
             os.dup2(se.fileno(), sys.stderr.fileno())
 
-    global loop, stop_loop, loop_thread
     loop, stop_loop, loop_thread = create_and_start_event_loop()
 
     try: