Use monkeypatch to ensure os.system is restored

Without this patch, running tests like test_edit_number before
test_default_to_shell_good in the same process will trigger
this error:

```
_______________________________________________________________ test_default_to_shell_good ________________________________________________________________

capsys = <_pytest.capture.CaptureFixture object at 0x7f7e90103898>

    def test_default_to_shell_good(capsys):
        app = cmd2.Cmd()
        app.default_to_shell = True
        line = 'ls'
        statement = app.parser_manager.parsed(line)
        retval = app._default(statement)
        assert not retval
        out, err = capsys.readouterr()
>       assert out == ''
E       AssertionError: assert '*** Unknown syntax: ls \n' == ''
E         - *** Unknown syntax: ls

tests/test_cmd2.py:840: AssertionError
```

Using monkeypatch fixture to restore os.system fixes the problem here.

1 files changed, 8 insertions, 8 deletions

diff --git a/tests/test_cmd2.py b/tests/test_cmd2.py
index 1deff479..0e25529a 100644
--- a/tests/test_cmd2.py
+++ b/tests/test_cmd2.py
@@ -661,13 +661,13 @@ def test_edit_no_editor(base_app, capsys):
     expected = _expected_no_editor_error()
     assert normalize(str(err)) == expected
 
-def test_edit_file(base_app, request):
+def test_edit_file(base_app, request, monkeypatch):
     # Set a fake editor just to make sure we have one.  We aren't really going to call it due to the mock
     base_app.editor = 'fooedit'
 
     # Mock out the os.system call so we don't actually open an editor
     m = mock.MagicMock(name='system')
-    os.system = m
+    monkeypatch.setattr("os.system", m)
 
     test_dir = os.path.dirname(request.module.__file__)
     filename = os.path.join(test_dir, 'script.txt')
@@ -677,13 +677,13 @@ def test_edit_file(base_app, request):
     # We think we have an editor, so should expect a system call
     m.assert_called_once_with('{} {}'.format(base_app.editor, filename))
 
-def test_edit_number(base_app):
+def test_edit_number(base_app, monkeypatch):
     # Set a fake editor just to make sure we have one.  We aren't really going to call it due to the mock
     base_app.editor = 'fooedit'
 
     # Mock out the os.system call so we don't actually open an editor
     m = mock.MagicMock(name='system')
-    os.system = m
+    monkeypatch.setattr("os.system", m)
 
     # Run help command just so we have a command in history
     run_cmd(base_app, 'help')
@@ -693,13 +693,13 @@ def test_edit_number(base_app):
     # We have an editor, so should expect a system call
     m.assert_called_once()
 
-def test_edit_blank(base_app):
+def test_edit_blank(base_app, monkeypatch):
     # Set a fake editor just to make sure we have one.  We aren't really going to call it due to the mock
     base_app.editor = 'fooedit'
 
     # Mock out the os.system call so we don't actually open an editor
     m = mock.MagicMock(name='system')
-    os.system = m
+    monkeypatch.setattr("os.system", m)
 
     # Run help command just so we have a command in history
     run_cmd(base_app, 'help')
@@ -1222,7 +1222,7 @@ def abbrev_app():
     app.stdout = StdOut()
     return app
 
-def test_exclude_from_history(abbrev_app):
+def test_exclude_from_history(abbrev_app, monkeypatch):
     # Run all variants of run
     run_cmd(abbrev_app, 'run')
     run_cmd(abbrev_app, 'ru')
@@ -1230,7 +1230,7 @@ def test_exclude_from_history(abbrev_app):
 
     # Mock out the os.system call so we don't actually open an editor
     m = mock.MagicMock(name='system')
-    os.system = m
+    monkeypatch.setattr("os.system", m)
 
     # Run all variants of edit
     run_cmd(abbrev_app, 'edit')