pkgng: add test prepare task

Make sure test environment does not contain the
test package.

tests/integration/targets/pkgng/tasks/freebsd.yml

@@ -1,4 +1,12 @@
 ---
+##
+## pkgng - prepare test environment
+##
+- name: Remove test package
+  pkgng:
+    name: '{{ pkgng_test_pkg_name }}'
+    state: absent
+
 ##
 ## pkgng - example - state=present for single package
 ##