Restore pexpect tests and bypass isalive/wait on macOS. Ref #2022.

_pytest/pytester.py

@@ -1002,8 +1002,6 @@ class Testdir:
         pexpect = pytest.importorskip("pexpect", "3.0")
         if hasattr(sys, 'pypy_version_info') and '64' in platform.machine():
             pytest.skip("pypy-64 bit not supported")
-        if sys.platform == "darwin":
-            pytest.xfail("pexpect does not work reliably on darwin?!")
         if sys.platform.startswith("freebsd"):
             pytest.xfail("pexpect does not work reliably on freebsd")
         logfile = self.tmpdir.join("spawn.out").open("wb")

testing/test_pdb.py

@@ -1,4 +1,5 @@
 import sys
+import platform
 
 import _pytest._code
 import pytest
@@ -80,6 +81,8 @@ class TestPDB:
 
     @staticmethod
     def flush(child):
+        if platform.system() == 'Darwin':
+            return
         if child.isalive():
             child.wait()