support/testing/tests/package/test_shadow: fix flake8 warning

Fixes:

support/testing/tests/package/test_shadow.py:3:1: F401 'infra.basetest.BASIC_TOOLCHAIN_CONFIG' imported but unused

Reported by https://gitlab.com/buildroot.org/buildroot/-/jobs/4643484762.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/testing/tests/package/test_shadow.py

@@ -1,6 +1,6 @@
 import os
 
-from infra.basetest import BRTest, BASIC_TOOLCHAIN_CONFIG
+from infra.basetest import BRTest
 
 
 class TestShadow(BRTest):