Remove nondeterminism from expansion tests

[damccorm]

These depend on https://github.com/apache/beam/blob/47740d07ed9e0824b4975ca4cadece74aae38302/sdks/python/apache_beam/transforms/external.py#L1007 which can lead to errors like:

    def test_classpath(self):
      with tempfile.TemporaryDirectory() as temp_dir:
        try:
          # Avoid having to prefix everything in our test strings.
          oldwd = os.getcwd()
          os.chdir(temp_dir)
          # Touch some files for globing.
          with open('a1.jar', 'w') as _:
            pass

          service = JavaJarExpansionService(
              'main.jar', classpath=['a*.jar', 'b.jar'])
>         self.assertEqual(
              service._default_args(),
              ['{{PORT}}', '--filesToStage=main.jar,a1.jar,b.jar'])
E             AssertionError: Lists differ: ['{{P[13 chars]lesToStage=main.jar,a1.jar,b.jar', '--alsoStartLoopbackWorker'] != ['{{P[13 chars]lesToStage=main.jar,a1.jar,b.jar']
E             
E             First list contains 1 additional elements.
E             First extra element 2:
E             '--alsoStartLoopbackWorker'
E             
E             - ['{{PORT}}',
E             -  '--filesToStage=main.jar,a1.jar,b.jar',
E             ?                                        ^
E             
E             + ['{{PORT}}', '--filesToStage=main.jar,a1.jar,b.jar']
E             ? ++++++++++++                                       ^
E             
E             -  '--alsoStartLoopbackWorker']

and overall flakiness like https://github.com/apache/beam/actions/workflows/beam_PreCommit_Python_Coverage.yml?query=event%3Aschedule

This removes the nondeterminism by clearing the cache.

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• [ ] Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• [ ] Update CHANGES.md with noteworthy changes.
• [ ] If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI or the workflows README to see a list of phrases to trigger workflows.

[damccorm]

R: @ahmedabu98 @jrmccluskey

[github-actions[bot]]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control. If you'd like to restart, comment assign set of reviewers