Fix error in run_pgaudit_test()

Initially command `test was missing`, but after it was added
test failed and crashed in a way that no more tests were run.
This behavior is known issue tracked here in issue #537

Problem is that `DOCKER_EXTRA_ARGS` variable is set already
which it shouldn't be, so the `test` command returns 0,
so no `cp` happens and we get the crash.

Variable is empty, but apparently do exist as
`test -v DOCKER_EXTRA_ARGS` is `TRUE`.

Introduced by PR #399

test/run_test

@@ -934,7 +934,7 @@ run_pgaudit_test()
   # create a dir for config
   config_dir=$(mktemp -d --tmpdir pg-hook-volume.XXXXX)
   add_cleanup_command /bin/rm -rf "$config_dir"
-  -v DOCKER_EXTRA_ARGS || cp -r "$test_dir"/examples/pgaudit/* "$config_dir"
+  test -n "$DOCKER_EXTRA_ARGS" || cp -r "$test_dir"/examples/pgaudit/* "$config_dir"
   setfacl -R -m u:26:rwx "$config_dir"
 
   # create a dir for data