Fix my dumb mistake merging run with context code

Author: Maurizio Branca

test/common.py

@@ -14,8 +14,6 @@
 # a commercial license, send an email to license@arduino.cc.
 import collections
 import os
-import pytest
-import invoke
 
 from invoke.context import Context
 
@@ -50,6 +48,6 @@ def _run(cmd_string):
         cli_full_line = "{} {}".format(cli_path, cmd_string)
         run_context = Context()
         with run_context.cd(working_dir):
-            return invoke.run(cli_full_line, echo=False, hide=True, warn=True, env=env)
+            return run_context.run(cli_full_line, echo=False, hide=True, warn=True, env=env)
 
     return _run

test/conftest.py

@@ -77,7 +77,10 @@ def _run_session_command(pytestconfig, tmpdir_factory, downloads_dir):
         "ARDUINO_DOWNLOADS_DIR": downloads_dir,
         "ARDUINO_SKETCHBOOK_DIR": data_dir,
     }
-    working_dir = tmpdir_factory.mktemp("SessionTestWork")
+    # it looks like the pyinvoke library has a few problems in dealing with the path
+    # object, so to avoid this issue we can convert them to str.
+    # Take a look at https://github.com/pyinvoke/invoke/issues/454 for more details.
+    working_dir = str(tmpdir_factory.mktemp("SessionTestWork"))
 
     return build_runner(cli_path, env, working_dir)