run-tests: flush output stream before prompting to accept changes

With py3 on Windows, the prompt to accept changes in an --interactive run wasn't
showing up until after the choice was read from stdin.

tests/run-tests.py

@@ -1875,6 +1875,7 b' class TestResult(unittest._TextTestResul'
                         'changes)')
                 else:
                     self.stream.write('Accept this change? [n] ')
+                    self.stream.flush()
                     answer = sys.stdin.readline().strip()
                     if answer.lower() in ('y', 'yes'):
                         if test.path.endswith(b'.t'):