testproc: Pass through full system environment

Let's just rely on tox for environment isolation, otherwise we just
break stuff on Windows.

tests/end2end/fixtures/testprocess.py

@@ -251,20 +251,11 @@ class Process(QObject):
         if args is None:
             args = self._default_args()
 
-        if env is None:
-            procenv = QProcessEnvironment.systemEnvironment()
-        else:
-            procenv = QProcessEnvironment()
+        procenv = QProcessEnvironment.systemEnvironment()
+        if env is not None:
             for k, v in env.items():
                 procenv.insert(k, v)
 
-            # so --no-xvfb and Qt on Windows works
-            passthrough_vars = ['DISPLAY', 'HOME',
-                                'QT_QPA_PLATFORM_PLUGIN_PATH']
-            for var in passthrough_vars:
-                if var in os.environ:
-                    procenv.insert(var, os.environ[var])
-
         self.proc.readyRead.connect(self.read_log)
         self.proc.setProcessEnvironment(procenv)
         self.proc.start(executable, exec_args + args)