use flush in order to avoid hanging on mac

[brianmaissy]

Fixes #2022

[coveralls]

Coverage remained the same at 92.558% when pulling 0a0d97a on brianmaissy:pexpect_test_hangs_on_mac into 3181718 on pytest-dev:master.

[nicoddemus]

I don't have a Mac to check this myself, but I trust @brianmaissy has tested it himself and the change in the PR works. 😁

[blueyed]

Well, #2022 indicated that it should rather read everything (non-blocking) instead of calling wait().

Currently it causes tests to not have a returncode then. Working on that in #5313.

[blueyed]

Yeah, it turns out that it should read() here before wait()ing.
#5313.