Improve quitting from pdb

Regarding tests: it merges ``test_pdb_interaction``,
``test_pdb_print_captured_stdout``, and
``test_pdb_print_captured_stderr`` into
``test_pdb_print_captured_stdout_and_stderr`` (clarity and performance,
especially since pexpect tests are slow).

src/_pytest/debugging.py

@@ -118,6 +118,10 @@ class pytestPDB(object):
 
                 do_c = do_cont = do_continue
 
+                def set_quit(self):
+                    super(_PdbWrapper, self).set_quit()
+                    outcomes.exit("Quitting debugger")
+
                 def setup(self, f, tb):
                     """Suspend on setup().
 
@@ -210,8 +214,7 @@ def _enter_pdb(node, excinfo, rep):
     tw.sep(">", "entering PDB")
     tb = _postmortem_traceback(excinfo)
     rep._pdbshown = True
-    if post_mortem(tb):
-        outcomes.exit("Quitting debugger")
+    post_mortem(tb)
     return rep
 
 
@@ -242,4 +245,5 @@ def post_mortem(t):
     p = Pdb()
     p.reset()
     p.interaction(None, t)
-    return p.quitting
+    if p.quitting:
+        outcomes.exit("Quitting debugger")

src/_pytest/outcomes.py

@@ -49,13 +49,13 @@ class Failed(OutcomeException):
     __module__ = "builtins"
 
 
-class Exit(SystemExit):
+class Exit(Exception):
     """ raised for immediate program exits (no tracebacks/summaries)"""
 
     def __init__(self, msg="unknown reason", returncode=None):
         self.msg = msg
         self.returncode = returncode
-        SystemExit.__init__(self, msg)
+        super(Exit, self).__init__(msg)
 
 
 # exposed helper methods
@@ -63,7 +63,7 @@ class Exit(SystemExit):
 
 def exit(msg, returncode=None):
     """
-    Exit testing process as if SystemExit was triggered.
+    Exit testing process.
 
     :param str msg: message to display upon exit.
     :param int returncode: return code to be used when exiting pytest.