Add _sys_snapshot fixture and use it with more tests

src/_pytest/pytester.py

@@ -335,6 +335,15 @@ def testdir(request, tmpdir_factory):
     return Testdir(request, tmpdir_factory)
 
 
+@pytest.fixture
+def _sys_snapshot():
+    snappaths = SysPathsSnapshot()
+    snapmods = SysModulesSnapshot()
+    yield
+    snapmods.restore()
+    snappaths.restore()
+
+
 @pytest.fixture
 def _config_for_test():
     from _pytest.config import get_config