docker: Extend tests to run a very basic Zeek script

Mostly because a `-v` shortcuts quite, while a zeek -e 'print zeek_version()'
will also parse all base scripts, etc.

docker/btest/Baseline/docker.structure_tests/output

@@ -1,5 +1,6 @@
 ### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
 zeek version xxx
+zeek version xxx
 XXX
 [sources]
 zeek = https://github.com/zeek/packages

docker/btest/docker/structure_tests.sh

@@ -7,6 +7,9 @@ TEST_TAG=${TEST_TAG:-zeek:latest}
 # Check that `zeek` can be run.
 docker run --rm "${TEST_TAG}" zeek -v | sed 's/\(zeek version\) .*/\1 xxx/'
 
+# ...and load and execute some basic scripts, too.
+docker run --rm "${TEST_TAG}" zeek -e 'print fmt("zeek version %s", zeek_version())' | sed 's/\(zeek version\) .*/\1 xxx/'
+
 # Check that this is a release build.
 docker run --rm "${TEST_TAG}" zeek-config --build_type | grep -q 'release'