support/testing: test_rust.py: fix flake8 errors

Fix flake8 errors introduced in commit f0d1f21195

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

(cherry picked from commit 12588a667a6e4d3c852a82a9d89c58bcf43eea1e)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/testing/tests/package/test_rust.py

@@ -1,7 +1,4 @@
 import os
-import tempfile
-import subprocess
-import shutil
 
 import infra.basetest
 
@@ -15,6 +12,7 @@ class TestRustBase(infra.basetest.BRTest):
                            options=["-initrd", img])
         self.emulator.login()
 
+
 class TestRustBin(TestRustBase):
     config = \
         """