chore: remove local test_utils directory

[plamut]

Closes #88. 🦕

Test utils are now available in their own repo, meaning that we can remove the local copy of it.

PR checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[HemangChothani]

LGTM!
System tests failed which is not relevant to changes.

[plamut]

I noticed that system test failure on another branch, too, but thought it has something to do with changes I was making to how BQ Storage client is used. I now see that it can also happen on master with a few unrelated modifications.

I cannot reproduce it locally, however, I only observed it on Kokoro. I restarted the checks, and if they fail again for the same reason, I will submit the workaround from another branch in a new PR.

Edit: The failure did not repeat, meaning that the test is flaky, but not a blocker. We can wait with the fix to be merged in #55.