pdb: resume capturing after continue

After `pdb.set_trace()` capturing is turned off.
This patch resumes it after using the `continue` (or `c` / `cont`)
command.

_pytest/debugging.py

@@ -59,7 +59,23 @@ def set_trace(cls):
             tw.line()
             tw.sep(">", "PDB set_trace (IO-capturing turned off)")
             cls._pluginmanager.hook.pytest_enter_pdb(config=cls._config)
-        cls._pdb_cls().set_trace(frame)
+
+            class PytestPdb(cls._pdb_cls):
+                def do_continue(self, arg):
+                    ret = super(PytestPdb, self).do_continue(arg)
+                    if self._pytest_capman:
+                        tw = _pytest.config.create_terminal_writer(cls._config)
+                        tw.line()
+                        tw.sep(">", "PDB continue (IO-capturing resumed)")
+                        self._pytest_capman.resumecapture()
+                    return ret
+                do_c = do_cont = do_continue
+
+            _pdb = PytestPdb()
+            _pdb._pytest_capman = capman
+        else:
+            _pdb = cls._pdb_cls()
+        _pdb.set_trace(frame)
 
 
 class PdbInvoke: