meson: Flush stdout in testwrap

Otherwise the progress won't reliably be displayed during a test. Reviewed-by: Noah Misch <noah@leadboat.com> Discussion: https://postgr.es/m/kx6xu7suexal5vwsxpy7ybgkcznx6hgywbuhkr6qabcwxjqax2@i4pcpk75jvaa Backpatch-through: 16

Author: anarazel

src/tools/testwrap

@@ -61,6 +61,7 @@ for line in sp.stdout:
  if line.startswith(b'ok '):
  line = line.replace(b' # TODO ', b' # testwrap-overridden-TODO ', 1)
  sys.stdout.buffer.write(line)
+ sys.stdout.flush()
 returncode = sp.wait()
 
 if returncode == 0: