streamline pytester API majorly:

- integrate conftest into pytester plugin
- introduce runpytest() to either call runpytest_inline (default) or
  runpytest_subprocess (python -m pytest)
- move testdir.inline_runsource1 to pdb tests
- strike some unneccessary methods.
- a new section "writing plugins" and some better pytester docs

--HG--
branch : testrefactor

testing/test_assertrewrite.py

@@ -468,12 +468,12 @@ def test_rewritten():
         tmp = "--basetemp=%s" % p
         monkeypatch.setenv("PYTHONOPTIMIZE", "2")
         monkeypatch.delenv("PYTHONDONTWRITEBYTECODE", raising=False)
-        assert testdir.runpybin("py.test", tmp).ret == 0
+        assert testdir.runpytest_subprocess(tmp).ret == 0
         tagged = "test_pyc_vs_pyo." + PYTEST_TAG
         assert tagged + ".pyo" in os.listdir("__pycache__")
         monkeypatch.undo()
         monkeypatch.delenv("PYTHONDONTWRITEBYTECODE", raising=False)
-        assert testdir.runpybin("py.test", tmp).ret == 1
+        assert testdir.runpytest_subprocess(tmp).ret == 1
         assert tagged + ".pyc" in os.listdir("__pycache__")
 
     def test_package(self, testdir):