Pick up a panic fix of f1bd0923b8
Some CirleCI builds fail somewhat mysteriously, e.g. https://circleci.com/gh/hashicorp/nomad/52676 , due to this bug. The test json file emit the following:
```
{"Time":"2020-03-28T12:03:08.602544522Z","Action":"output","Package":"github.com/hashicorp/nomad/client/allocrunner","Test":"TestGroupServiceHook_Update08Alloc","Output":"panic: send on closed channel\n"}
{"Time":"2020-03-28T12:03:08.602576075Z","Action":"output","Package":"github.com/hashicorp/nomad/client/allocrunner","Test":"TestGroupServiceHook_Update08Alloc","Output":"\n"}
{"Time":"2020-03-28T12:03:08.602584429Z","Action":"output","Package":"github.com/hashicorp/nomad/client/allocrunner","Test":"TestGroupServiceHook_Update08Alloc","Output":"goroutine 403 [running]:\n"}
{"Time":"2020-03-28T12:03:08.602590561Z","Action":"output","Package":"github.com/hashicorp/nomad/client/allocrunner","Test":"TestGroupServiceHook_Update08Alloc","Output":"github.com/hashicorp/nomad/vendor/github.com/stretchr/testify/assert.Eventually.func1(0xc000a78000, 0xc0009cf160)\n"}
{"Time":"2020-03-28T12:03:08.602598464Z","Action":"output","Package":"github.com/hashicorp/nomad/client/allocrunner","Test":"TestGroupServiceHook_Update08Alloc","Output":"\t/home/circleci/go/src/github.com/hashicorp/nomad/vendor/github.com/stretchr/testify/assert/assertions.go:1494 +0x47\n"}
{"Time":"2020-03-28T12:03:08.602604952Z","Action":"output","Package":"github.com/hashicorp/nomad/client/allocrunner","Test":"TestGroupServiceHook_Update08Alloc","Output":"created by github.com/hashicorp/nomad/vendor/github.com/stretchr/testify/assert.Eventually\n"}
```
