Fix #1549 - flush after each line of logs.

Includes some refactoring of log_printer_test to support checking for flush(), and so that each test calls the unit-under-test directly, instead of through a helper function.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

compose/cli/log_printer.py

@@ -26,6 +26,7 @@ class LogPrinter(object):
         generators = list(self._make_log_generators(self.monochrome, prefix_width))
         for line in Multiplexer(generators).loop():
             self.output.write(line)
+            self.output.flush()
 
     def _make_log_generators(self, monochrome, prefix_width):
         def no_color(text):