Hum, test_executable() of test_venv is skipped if run in a venv, but not test_multiprocessing():

    # If a venv is created from a source build and that venv is used to
    # run the test, the pyvenv.cfg in the venv created in the test will
    # point to the venv being used to run the test, and we lose the link
    # to the source build - so Python can't initialise properly.
    @skipInVenv
        ...
        self.run_with_capture(venv.create, self.env_dir)
        envpy = os.path.join(os.path.realpath(self.env_dir),
                             self.bindir, self.exe)
        ...

    def test_multiprocessing(self):
        ...
        self.run_with_capture(venv.create, self.env_dir)
        envpy = os.path.join(os.path.realpath(self.env_dir),
                             self.bindir, self.exe)
        ...

The test has been added by:

commit 4e02f8f8b4baab63f927cfd87b401200ba2969e9
Author: Steve Dower <steve.dower@microsoft.com>
Date:   Fri Jan 25 14:59:12 2019 -0800