kubens add a short-circuit to bypass API call for tests

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

test/kubens.bats

@@ -1,8 +1,13 @@
 #!/usr/bin/env bats
 
 COMMAND="${COMMAND:-$BATS_TEST_DIRNAME/../kubens}"
+
+# TODO(ahmetb) remove this after bash implementations are deleted
 export KUBECTL="$BATS_TEST_DIRNAME/../test/mock-kubectl"
 
+# short-circuit namespace querying in kubens go implementation
+export _MOCK_NAMESPACES=1
+
 load common
 
 @test "--help should not fail" {