Unit tests should not depend on their running environment

[Chris MacNaughton]

When the environment exports environment variables that are mocked in unit testing (eg: already defining ARCH: https://github.com/openstack/diskimage-builder/blob/2e6a7f949c19ca81e545592462ab393ff5575245/diskimage_builder/elements/package-installs/tests/test_package_squash.py#L76), the tests fail because:

Failed to import test module: diskimage_builder.elements.package-installs.tests.test_package_squash
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/unittest2/loader.py", line 456, in _find_test_path
    module = self._get_module_from_name(name)
  File "/usr/lib/python3/dist-packages/unittest2/loader.py", line 395, in _get_module_from_name
    __import__(name)
  File "/build/python-diskimage-builder-3.0.0/diskimage_builder/elements/package-installs/tests/test_package_squash.py", line 53, in <module>
    class TestPackageInstall(base.BaseTestCase):
  File "/build/python-diskimage-builder-3.0.0/diskimage_builder/elements/package-installs/tests/test_package_squash.py", line 76, in TestPacka
geInstall
    @mock.patch.object(os, 'environ', dict(ARCH='arm64', **os.environ))
TypeError: type object got multiple values for keyword argument 'ARCH'

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/735309

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/735309
Committed: https://git.openstack.org/cgit/openstack/diskimage-builder/commit/?id=8f57ed9b9ed3e4dcb348487b29c18d2772147e07
Submitter: Zuul
Branch: master

commit 8f57ed9b9ed3e4dcb348487b29c18d2772147e07
Author: Chris MacNaughton <email address hidden>
Date: Fri Jun 12 10:37:20 2020 +0200

    Stop bringing the test environment into the mocks

    Change-Id: I1780f186cf107242cacd6d3da7a5bc81a330b536
    Closes-Bug: #1883224