support/testing: test_rust.py: fix flake8 errors

Fix flake8 errors introduced in commit f0d1f211

Fixes:
support/testing/tests/package/test_rust.py:2:1: F401 'tempfile' imported but unused
support/testing/tests/package/test_rust.py:3:1: F401 'subprocess' imported but unused
support/testing/tests/package/test_rust.py:4:1: F401 'shutil' imported but unused
support/testing/tests/package/test_rust.py:18:1: E302 expected 2 blank lines, found 1