add cli tests for kubens (#117)

* split bats test invocation by executable

* add more cli tests for kubens

* clean up kubens tests

* small cleanup to kubens tests

test/mock-kubectl

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+[[ -n $DEBUG ]] && set -x
+
+set -eou pipefail
+
+if [[ $@ == *'get namespaces'* ]]; then
+  echo "ns1"
+  echo "ns2"
+else
+  kubectl $@
+fi