test-runner: use may_block=True for context iteration (and move location)

This allows the callers condition to be checked immediately without
the mainloop running. In addition may_block=True allows the mainloop
to poll/sleep rather than immediately return back to the caller. This
handles async IO much better than may_block=False, at least for our
use-case.

tools/run-tests

@@ -722,8 +722,6 @@ class Namespace:
 		context = mainloop.get_context()
 
 		while True:
-			context.iteration(may_block=False)
-
 			try:
 				ret = func(*args)
 				if ret:
@@ -735,8 +733,6 @@ class Namespace:
 					GLib.source_remove(timeout)
 				raise e
 
-			sleep(0.1)
-
 			if done.value == True:
 				if isinstance(exception, Exception):
 					raise exception
@@ -745,6 +741,8 @@ class Namespace:
 				else:
 					return
 
+			context.iteration(may_block=True)
+
 	def __str__(self):
 		ret = 'Namespace: %s\n' % self.name
 		ret += 'Processes:\n'