Removed stale google-cls-v2 related tests

[kvo3]

Remove stale google-cls-v2 tests (commit 0025a5e)
google-cls-v2 was already dropped from the --provider CLI choices, so --provider google-cls-v2 fails immediately with invalid choice. Nine test files still targeted it and could never run:

• Deleted 9 dead integration test files: unit_flags.google-cls-v2.sh and 8 e2e_*.google-cls-v2.sh files.
• test/run_tests.sh: removed google-cls-v2 from the default/fallback provider lists and deleted the now-dead case branches for the removed files.
• test/integration/test_setup.sh: removed the google-cls-v2→google-batch override shim and the dsub_google-cls-v2/dstat_google-cls-v2/ddel_google-cls-v2 functions, which only existed to support the deleted files.
• test/integration/test_setup_e2e.py: removed dsub_google_cls_v2().
• test/integration/e2e_python_api.py: removed the google_cls_v2 import and provider branch.
• The google-cls-v2 provider implementation itself (dsub/providers/google_cls_v2.py, etc.) and test/unit/batch_handling_test.py were left untouched — no CLI path reaches them, and neither is broken by this change.

Fix 2 remaining dry-run failures (commit 0157199)

• unit_flags.test-fails.sh: replaced /bin/true with the portable true builtin — /bin/true doesn't exist on macOS, so bash failed the fallback (|| /bin/true) before the test's own assertions ever ran.
• test/integration/test_unit_util.sh: fixed assert_output_empty/assert_err_empty, which tried to execute their own failure message as a shell command ([[ ... ]] || "Assert: ...") instead of printing it, producing a confusing "command not found" instead of a real assertion failure.
• test/integration/unit_version.sh: filters out known-benign third-party warnings (urllib3's NotOpenSSLWarning, google-cloud-batch's Python-version FutureWarning) before asserting stderr is otherwise empty, since some environments legitimately emit these on every dsub invocation.