Use a modern fixture instead of removed cached_setup (pytest 4.1)

This require us to remove the --gwscope option because we won't
be able to parametrize the fixture based on the deprecated
'pytest.config' variable

Also remove pytest_report_header: it was displaying the removed "scope" option
and execnet's version, which seems odd and not very useful nowadays

WIP

Author: nicoddemus

testing/conftest.py

@@ -39,10 +39,6 @@ def pytest_addoption(parser):
     group.addoption(
         '--gx', action="append", dest="gspecs", default=None,
         help="add a global test environment, XSpec-syntax. ")
-    group.addoption(
-        '--gwscope', action="store", dest="scope", default="session",
-        type="choice", choices=["session", "function"],
-        help="set gateway setup scope, default: session.")
     group.addoption(
         '--pypy', action="store_true", dest="pypy",
         help="run some tests also against pypy")
@@ -52,13 +48,6 @@ def pytest_addoption(parser):
               "page on invalid addresses"))
 
 
-def pytest_report_header(config):
-    return [
-        "gateway test setup scope: %s" % config.getvalue("scope"),
-        "execnet: {} -- {}".format(execnet.__file__, execnet.__version__),
-    ]
-
-
 @pytest.fixture
 def specssh(request):
     return getspecssh(request.config)
@@ -151,15 +140,15 @@ def anypython(request):
     return executable
 
 
+@pytest.fixture(scope='session')
+def group():
+    g = execnet.Group()
+    yield g
+    g.terminate(timeout=1)
+
+
 @pytest.fixture
-def gw(request, execmodel):
-    scope = request.config.option.scope
-    group = request.cached_setup(
-        setup=execnet.Group,
-        teardown=lambda group: group.terminate(timeout=1),
-        extrakey="testgroup",
-        scope=scope,
-    )
+def gw(request, execmodel, group):
     try:
         return group[request.param]
     except KeyError:
@@ -188,6 +177,8 @@ def gw(request, execmodel):
             group.makegateway('popen//id=proxy-transport')
             gw = group.makegateway('popen//via=proxy-transport//id=proxy'
                                    '//execmodel=%s' % execmodel.backend)
+        else:
+            assert 0, "unknown execmodel: {}".format(request.param)
         return gw