Add Python Broker module load test to Docker image btests

We missed the fact that the broker module didn't load due to a wrong libpython
version because we didn't test that load.

Also remove an unused environment variable from btest.cfg.

docker/btest/btest.cfg

@@ -9,5 +9,4 @@ MinVersion  = 0.63
 [environment]
 LC_ALL=C
 PATH=%(testbase)s/../../auxil/btest:%(default_path)s
-TEST_IMAGE=${TEST_TAG:-zeek:latest}
 TZ=UTC

docker/btest/docker/structure_tests.sh

@@ -18,3 +18,6 @@ docker run --rm "${TEST_TAG}" zkg config
 
 # Check that a plugin can be installed. We pick any plugin with minimal deps here.
 docker run --rm "${TEST_TAG}" zkg install --force sethhall/domain-tld | sed 's/(.*)/(XXX)/'
+
+# Check that the Broker Python module loads
+docker run --rm "${TEST_TAG}" python3 -c "import broker"