test: Use seperate test projects for bigframes integration tests

[TrevorBergeron]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request adds the BIGFRAMES_TEST_PROJECT environment variable to several Kokoro configuration files, distinguishing between bigframes-load-testing for continuous jobs and bigframes-testing for presubmit jobs. I have no feedback to provide.