pyln-testing: improve wait_for a bit

This 'fixes' the `wait_for` helper by removing a pointless final
`time.sleep()`, thus potentially making the method return quicker.

The old code could have had three final states:
 - success() := True
 - Timeout and success() := True
 - Timeout and success() := False

The new code has just two final state:
 - success() := True
 - Timeout and success() := False

It ensures the final `time.sleep()` is just the right amount before timeout.
And more importantly making it more readable :-)

Changelog-None

contrib/pyln-testing/pyln/testing/utils.py

@@ -84,13 +84,14 @@ def env(name, default=None):
 def wait_for(success, timeout=TIMEOUT):
     start_time = time.time()
     interval = 0.25
-    while not success() and time.time() < start_time + timeout:
-        time.sleep(interval)
+    while not success():
+        time_left = start_time + timeout - time.time()
+        if time_left <= 0:
+            raise ValueError("Timeout while waiting for {}", success)
+        time.sleep(min(interval, time_left))
         interval *= 2
         if interval > 5:
             interval = 5
-    if time.time() > start_time + timeout:
-        raise ValueError("Error waiting for {}", success)
 
 
 def write_config(filename, opts, regtest_opts=None, section_name='regtest'):