Add tests for every implemented command

tests/service_create.bats

@@ -0,0 +1,12 @@
+#!/usr/bin/env bats
+load test_helper
+
+@test "($PLUGIN_COMMAND_PREFIX:create) success" {
+  run dokku "$PLUGIN_COMMAND_PREFIX:create" l
+  assert_contains "${lines[*]}" "container created: l"
+}
+
+@test "($PLUGIN_COMMAND_PREFIX:create) error when there are no arguments" {
+  run dokku "$PLUGIN_COMMAND_PREFIX:create"
+  assert_contains "${lines[*]}" "Please specify a name for the service"
+}