pexpect tests hang on macOS

[jaraco]

In #985, I discovered that many tests on macOS are bypassed because pexpect appears to operate unreliably on that platform.

[jaraco]

I found that even consuming all of the input before calling .wait() doesn't stop the block forever behavior. But if instead I do something like the following, it causes child.isalive() to return False and the test to pass.

diff -r 1d9397a85c2c testing/test_pdb.py
--- a/testing/test_pdb.py   Fri Oct 21 01:36:24 2016 +0200
+++ b/testing/test_pdb.py   Fri Oct 21 12:01:25 2016 -0400
@@ -141,10 +141,20 @@
         child = testdir.spawn_pytest("--pdb %s" % p1)
         #child.expect(".*import pytest.*")
         child.expect("(Pdb)")
+        print("got here 1")
         child.sendeof()
+        print("got here 2")
         child.expect("1 error")
+        print("got here 3")
+        while True:
+            try:
+                child.read_nonblocking()
+            except Exception:
+                break
         if child.isalive():
+            print("got here 6")
             child.wait()
+        print("got here 5")

     def test_pdb_interaction_on_internal_error(self, testdir):
         testdir.makeconftest("""
$ python -m tox -e py35 -- -s -k test_pdb_interaction_on_collection_issue181 -v
GLOB sdist-make: /Users/jaraco/Dropbox/code/public/pytest/setup.py
py35 inst-nodeps: /Users/jaraco/Dropbox/code/public/pytest/.tox/dist/pytest-3.0.4.dev0.zip
py35 installed: hypothesis==3.5.3,mock==2.0.0,nose==1.3.7,pbr==1.10.0,pexpect==4.2.1,ptyprocess==0.5.1,py==1.4.31,pytest==3.0.4.dev0,requests==2.11.1,six==1.10.0,spark-parser==1.4.0,uncompyle6==2.9.2,xdis==3.1.0
py35 runtests: PYTHONHASHSEED='1622146962'
py35 runtests: commands[0] | pytest --lsof -rfsxX -s -k test_pdb_interaction_on_collection_issue181 -v
=========================================== test session starts ===========================================
platform darwin -- Python 3.5.2, pytest-3.0.4.dev, py-1.4.31, pluggy-0.4.0 -- /Users/jaraco/Dropbox/code/public/pytest/.tox/py35/bin/python3.5
cachedir: .cache
rootdir: /Users/jaraco/Dropbox/code/public/pytest, inifile: tox.ini
plugins: hypothesis-3.5.3
collected 1724 items 

testing/test_pdb.py::TestPDB::test_pdb_interaction_on_collection_issue181 got here 1
got here 2
got here 3
got here 5
PASSED

========================================== 1723 tests deselected ==========================================
================================ 1 passed, 1723 deselected in 2.42 seconds ================================
_________________________________________________ summary _________________________________________________
  py35: commands succeeded
  congratulations :)

[brianmaissy]

Reappeared in test_pdb_custom_cls_with_settrace in d75748e, I'll open a PR